Jurassic Trigoniida (Bivalvia) from Gebel Maghara, northern Sinai, Egypt

Non-technical Summary. On the Sinai Peninsula, a thick pile of terrestrial to shallow-marine sediments accumulated during the Jurassic period. The sea floor at that time was colonized chiefly by brachiopods and bivalves. Among the latter, trigoniids were a comparatively rare but diverse faunal element, represented by very small to small individuals. They belong to 14 species and nine genera. One of these species is new to science, but five genera with species occurring also elsewhere in Jurassic seas were recognized as new, all of them characterized by a very small to small size. Abstract. The Jurassic trigoniid bivalves of Gebel Maghara, northern Sinai, Egypt are described and figured. They belong to 14 species, nine genera, and two families. The identified taxa occur in rocks ranging in age from the Toarcian (Lower Jurassic) to the lower Kimmeridgian (Upper Jurassic). Five genera and one species are new: Magharitrigonia asymmetrica new genus new species; Cotswoldella aff. C. hemisphaerica (Lycett, 1853); Cornbrashella pullus (J. de C. Sowerby, 1826); Parorthotrigonia lepidomorpha (Abdallah and Fahmy, 1969); and Retetrigonia imbricata (J. de C. Sowerby, 1826). Five species, Trigonia reticulata Agassiz, 1840, T. cf. T. castor d'Orbigny, 1849, Promyophorella tuberculata (Agassiz, 1840), Orthotrigonia exortiva (Kitchin, 1903), and O. gracilis (Kitchin, 1903) are recorded from the Jurassic rocks of Gebel Maghara for the first time.


Introduction
Although trigoniid bivalves reached their greatest diversity during the Cretaceous period, they steadily increased in species richness during the Jurassic (Stanley, 1977;Kelly, 1995).Trigoniids became widely dispersed during the late Early Jurassic to early Middle Jurassic (Echevarría et al., 2021), reaching the European Tethys (Francis, 2000;Francis and Hallam, 2003) and the southeast Asian Tethys (Wandel, 1936;Hayami, 1972) by the Toarcian, the high latitudes of Antarctica by the Pliensbachian-Toarcian (Kelly, 1995), New Zealand by the Aalenian (Fleming, 1987), and eastern Africa by the Bajocian (Cox, 1965).
The Jurassic succession of Gebel Maghara is the thickest and most complete Jurassic succession not only of northern Sinai but also of the African-Arabian Plate (1800 m thick; Al Far, 1966;Hirsch, 1980;Keeley, 1994).Bivalves are the most abundant group in the Jurassic strata of Gebel Maghara, followed by gastropods, ammonites, brachiopods, corals, and echinoids.Ayoub-Hannaa et al. (2023) pointed out that the bivalves and gastropods of Gebel Maghara have been neglected from a taxonomic point of view since the pioneering study of Douvillé in the early twentieth century (Douvillé, 1916).Hirsch (1980), Abdelhamid (2002), and Khalil (2003) illustrated some bivalve and gastropod taxa from the area, but without a comprehensive taxonomic study.Abdelhady (2014) and Abdelhady andFürsich (2014, 2015a-c) listed 232 benthic and nektonic macrofaunal taxa from Gebel Maghara but did not perform a taxonomic analysis.The faunas are dominated by bivalves (60 species) and gastropods (52 species) (Abdelhady, 2014, appendix B).Therefore, Ayoub-Hannaa et al. (2017, 2023) began a detailed taxonomic revision of the bivalve fauna, dealing with protobranchs.The present study on the trigoniids is the second part of the comprehensive taxonomic study on the Jurassic bivalves of Gebel Maghara.In addition, the revision of the trigoniid fauna from the Jurassic of Egypt fills a gap in our knowledge of the Jurassic paleobiogeography of the group.1994).In northern Sinai, Jurassic strata crop out at Gebel Maghara, situated 50 km south of the Mediterranean shoreline, in an anticlinal structure that covers an area of about 400 km 2 , and represents the fill of a half-graben (Al Far, 1966;Keeley, 1994;Fig. 1.1-1.3).The oldest Jurassic rocks (Mashabba Formation) at Gebel Maghara are ?Pliensbachian to Toarcian in age and represent fluvial, marginal-marine, and shallow-marine environments (e.g., Ghandour and Fürsich, 2022).Subsequently, shallow-marine and carbonate sediments deposited on a homoclinal ramp prevailed up to the Kimmeridgian, except during the global early Bathonian sea-level fall (Haq et al., 1987) during which deltaic sandstones and coals of the Safa Formation (Al Far, 1966;Abdelhady and Fürsich, 2015a) accumulated.The basin was closed by compressional forces due to convergence between Eurasia and Africa at the Jurassic-Cretaceous transition (Abd El-Motaal and Kusky, 2003).

Material and methods
The studied trigonioid bivalve fauna consists of specimens collected by F.T. Fürsich in 1993 with complementary sampling by A.A. Abdelhady and F.T. Fürsich in 2012 from the Jurassic succession of Gebel Maghara, northern Sinai.Specimens were recovered from three sections: Gebel Homayir, Gebel Arousiah, and Gebel Mowerib.F.T. Fürsich collected additional specimens from the Toarcian Rajabiah and Shusha formations and the lower Kimmeridgian Masajid Formation of western Bir Maghara (section 4; Fig. 1.2).Abdelhady (2014) and Abdelhady and Fürsich (2015a-c) studied the litho-and biostratigraphy of the four sections, made taphonomic observations, and collected macrofossils bed by bed.Primary identification of the collected fossils took place during 2012-2014 as a part of a Ph.D. project by A.A. Abdelhady (2014).Faunal identification was greatly aided by the Jurassic bivalve catalogue at the GeoZentrum Nordbayern of Friedrich-Alexander-Universität, Erlangen.This catalogue contains photocopies of about 95% of all figured Jurassic bivalves worldwide.The trigonioid bivalves are mostly preserved with shell, are disarticulated, incomplete, and are small (H = 10 to ˂ 20 mm) to very small (H ˂ 10 mm) in size.Few specimens are preserved as composite molds.The hinge and other internal structures are usually covered with poorly indurated matrix, which was removed using diluted hydrogen peroxide (H 2 O 2 ).Specimens were cleaned in the laboratory and prepared mechanically using steel needles under a binocular microscope.In addition, an air abrasive was used to clean the flank costae, area, and teeth.The specimens were photographed after having been coated with ammonium chloride to enhance details of ornamental features.
The systematic classification of the bivalves is that of Carter et al. (2011).The morphological terminology follows Carter et al. (2012).As in Ayoub-Hannaa et al. (2023), the synonymy lists contain only references, which have been carefully checked by the authors, beginning with the original description of the particular taxon, and followed by literature records from the Jurassic of Egypt.More comprehensive synonymies can be found in the references cited.For abbreviations in synonymy lists and open nomenclature to indicate the degree of confidence in allocation of each entry see Matthews (1973) and Bengtson (1988).Using "cf." for some specimens refers to a provisional identification due to poor preservation, whereas a question mark indicates uncertain identification at the species level.
Linear measurements (taken with a Vernier caliper, accurate to 0.1 mm) are in millimeters.Measured dimensions, general morphologic terminology, teeth, and orientation are given in Figure 2. Abbreviations for dimensions are: L = length, H = height, I = inflation of articulated valves, and D = distance between umbo and anterior margin with the posterodorsal margin horizontal.
Repository and institutional abbreviation.-Allspecimens are housed in the collections of the Bayerische Staatssammlung für Paläontologie und Geologie in Munich under the prefix BSPG 2014 V.
Diagnosis.-Verysmall to small, subtrigonal, moderately to strongly inflated with orthogyrate to slightly opisthogyrate beak, a wide and smooth antecarinal sulcus, coarsely nodate escutcheon and marginal carinae; escutcheon small, cordate, and smooth; right and left valves of some species differently ornamented on areas; LV with narrow and shallow area, bipartite, divided into two unequal halves by a distinctly Occurrence.-UpperLower Jurassic (lower Toarcian) of Sinai, Egypt.
Etymology.-Combination of the type locality Gebel Maghara (Sinai, Egypt) and the genus Trigonia.
Posterior tooth (4b) narrower than anterior one and running parallel to posterodorsal margin.
Etymology.-Asymmetricus(Latin) = asymmetric; after the differently ornamented areas of the right and left valves.
Remarks.-Magharitrigonia asymmetrica n. gen.n. sp. is easily distinguished from other trigoniid species identified herein by its small to very small size (Table 1), a smooth and elongated-cordate escutcheon, a flank with few and widely spaced commarginal costae terminating in nodes close to antecarinal sulcus, a bipartite area to the left valve differing in ornamentation, and a tripartite area to the right valve with widely spaced commarginal costellae.The most closely related species is Magharitrigonia senex (Kobayashi andMori, 1954) n. gen. n. comb. (Kobayashi andMori, 1954, p. 167, pl. 16, fig. 8) from the Hettangian of Japan in having widely spaced commarginal costae with small spines close to the antecarinal sulcus, and a bipartite area of the left valve with reticulate ornamentation but differs in being larger and in having strongly convex anterior and ventral margins, a narrower antecarinal sulcus and a rounder valve.
Trigonia (Trigonia) patchamensis Fürsich and Heinze (1998, p. 155, pl. 2, figs. 6-10) from the Middle Jurassic strata of Kachchh, India, has an ornamented escutcheon, strongly rounded anterior and ventral margins, a smaller area that is divided into two parts by a median groove, and is less inflated.
Trigonia triangularis (Goldfuss, 1837), figured by the same authors (Pugaczewska, 1976, p. 85, pl. 26, figs. 1-5, pl. 20, figs. 1-2, 6-7, pl. 21, figs. 1, 4, 10) from the Middle Jurassic of Poland, and by Francis (2000, p. 84, pl. 7, figs. e-h) from the Bajocian of Germany is similar to the present species with respect to shell outline.Like M. asymmetrica n. gen.n. sp., it also has a wide and smooth antecarinal sulcus, widely spaced commarginal flank costae, and a small cordate and smooth escutcheon, but differs in having a blunt and strongly tuberculated marginal carina, the areas of the two valves having the same ornamentation (reticulated pattern), and in being larger than the present species.In addition, the ventral margin of T. triangularis is concave below the antecarinal sulcus, and strongly convex anteroventrally.
Remarks.-Cotswoldella aff.C. hemisphaerica n. gen.n. comb.can be easily separated from Jurassic Trigonia by its regular commarginal flank costae (Fig. 4.4, 4.6), very small escutcheon, faint marginal carina, and small area.The shape of the shell and the ornamentation of the flank of the present specimen resemble C. hemisphaerica (Lycett, 1853) n. gen.n. comb.figured by Lycett (1877) and Francis (2000) from the Middle Jurassic of England, but differs in having a narrower area with far fewer radial costellae that are not reticulated in the ventral part, a finer marginal carina, and flank costae that are narrower than the interspaces.For this reason, the specimen is referred to Lycett's species with qualification.Hirsch (1980) used the "race Asiatica" of Douvillé, 1916, as a separate species, Trigonia asiatica, on the basis of numerous commarginal flank costae, strong radial costellae of the area, a well-defined marginal carina, and its small size.
Etymology.-From the Cornbrash Formation, in which the type species is common.
Remarks.-Cornbrashella n. gen. is easily distinguished from Trigonia by its small size, trigonally ovate outline, transverse costellae to the escutcheon, and non-reticulate area with few radial costellae.The genus Neuquenitrigonia Leanza and Garate Zubillaga (1987, p. 209) is very similar, but the type species Trigonia huenickeni Leanza and Garate, 1985 (p. 290, fig. 3;pl. 1, figs. 1, 2) from the Bajocian of Argentina differs in being much larger, more inflated, and in having an acute apical angle, well-separated almost straight flank costae, and a larger escutcheon.
Description.-Shellvery small to small, triangular, inequilateral, slightly higher than long (Table 4), moderately inflated.Anterior and ventral margins convex, meeting in rounded curve.Posterodorsal margin slightly concave, meeting posterior margin at obtuse angle.Posterior margin relatively short.Umbo poorly inflated, triangular, located one-third of total valve length from anterior end.Beak sharply pointed, slightly opisthogyrate.Marginal carina well developed.Antecarinal sulcus smooth, narrow dorsally, widening towards posteroventral corner.Area wide, covered with radial, beaded costellae and crossed by fine commarginal growth lines (Fig. 4.9, 4.10).Escutcheon wide, depressed, cordate in Cornbrashella distincta (Kitchin, 1903) n. gen.n. comb.(Kitchin, 1903, p. 25, pl. 2, figs. 6, 7) from the Middle Jurassic of Kachchh, India, resembles the present species in having an ornamented escutcheon but differs in having an ovate valve, rounded margins, and its radial costellae of the area are fewer and less developed than in C. pullus n. gen.n. comb.Francis (2000, p. 91)  The general outline and ornamentation of the area is close to T. interlaevigata Quenstedt, 1857 (p. 503, pl. 67, figs. 7, 8) from the Middle Jurassic of Germany.That species is, however, larger and has a higher number of commarginal flank costae.
Remarks.-With respect to shell outline and ornamentation of the area and flank, the present material is very similar to Trigonia monilifera Agassiz, 1840 (p. 40, pl. 3, figs. 4-6) from the Upper Jurassic of France.Some authors such as Arkell (1930, p. 81-82) and Schneider et al. (2011, p. 263) regarded that species, together with T. papillata Agassiz, T. parvula Agassiz, and T. meriani Agassiz, as junior synonyms of T. reticulata Agassiz.This view is followed here.Schneider et al. (2011, p. 264) noted that T. reticulata is a highly variable species, in which the H/L ratio of the shells varies from 0.85 to 0.97, and thus shells are generally slightly longer than high.Although the present specimens are poorly preserved, they exhibit characteristic features of T. reticulata Agassiz, such as the strongly rounded anterior and ventral margins and reticulate area, except that they are smaller.
Remarks.-Trigonia castor d'Orbigny, 1849, can be easily distinguished from other Jurassic Trigonia species by its thick shell, large area, regular flank costae, and thick tuberculated marginal carina.The present specimen is incomplete and is, therefore, placed with reservation in T. castor, a species with similar shell outline and ornamentation of the area and flank.
Remarks.-Parorthotrigonia lepidomorpha (Abdallah and Fahmy, 1969) 7), in having subtriangular valves, rounded flank costae, and a smooth area.Because only lateral views of incomplete left valves are available, it is difficult to regard their specimens as belonging to P. lepidomorpha n. gen.n. comb.
Another similar species is Trigonia baylei Philippi, 1899 (p. 86, pl. 36, fig. 9) from the Upper Jurassic (Tithonian) of Chile.Although it also has smooth radial costae, it differs from the present species in being much more elongated (L > H) and in having wider intercostal spaces, a blunt marginal carina, and flank costae meeting the marginal carina almost at right angles (acute angles in the present species)."Trigonia" catenifera Hupé, 1854 (Philippi 1899, p. 85, pl. 36, fig. 5) from the Upper Jurassic of the same area has, in contrast to P. lepidomorpha n. gen.n. comb., costae that bifurcate towards the anteroventral margin and carry small tubercles on their crests (see Echevarría et al., 2021, fig. 18.3).
Diagnosis.-Verysmall myophorellid, outline pentagonal to subrounded, little inflated, inequilateral, with strongly rounded anterior and ventral margins, broad obliquely subtruncated posterior margin, straight posterodorsal margin, and a convex anterodorsal margin.Beaks very small, orthogyrate to slightly prosogyrate, area wide with widely spaced commarginal costellae passing over marginal carina onto flank.Median and escutcheon carinae absent; escutcheon very narrow and smooth; marginal carina distinct, sharp, and tuberculated; flank with radial costae crossed by commarginal costae forming a reticulate pattern with large spines at their intersections.
Remarks.-Retetrigonia n. gen.can be easily distinguished from other Jurassic genera by its reticulate flank ornamentation, wide area, lack of median and escutcheon carinae, a very narrow, smooth escutcheon, a subtruncated posterior margin, and small orthogyrate to slightly prosogyrate beaks.
With respect to shell outline and size, Retetrigonia n. gen. is somewhat similar to Ibotrigonia Kobayashi in Kobayashi and Tamura 1957, but that genus has a median carina and its flank is covered by widely spaced, tuberculated commarginal costae.
The wide antecarinal sulcus of the genus Frenguelliella Leanza, 1942 (type species: Trigonia inexspectata Jaworski, 1915, from the Pliensbachian of Argentina) distinguishes it from Retetrigonia.In addition, the flank and area of Frenguelliella is differently ornamented.
Remarks.-Characteristic features of Retetrigonia imbricata (J.de C. Sowerby, 1826) n. gen.n. comb.are its very small size, weak inflation, well-developed reticulate pattern of flank costellae with short spines at their intersections, wide and ornamented area, and the lack of median and escutcheon carinae.
The figured specimen of R. parcinoda (Lycett, 1872) n. gen.n. comb.(Lycett, 1872, p. 46, with text-fig.)from the Middle Jurassic of England strongly resembles the present species with respect to shell outline, size, and lack of median and escutcheon carina.Therefore, it probably falls in the variation of R. imbricata n. gen.n. comb.Although R. parcinoda n. gen.n. comb.Also has a reticulate flank ornamentation, it differs by its tiny spines at the intersections of radial and commarginal costae.
Remarks.- Kobayashi and Tamura (1955) erected the subgenus Promyophorella based on sharp costae carrying numerous small tubercles.They considered Promyophorella as ancestral to Myophorella s.s., but included it in that genus because they recognized numerous intermediate forms.Echevarría et al. (2021, p. 31) modified the diagnostic features listed by Kobayashi and Tamura (1955) to include a clearly opisthogyrate shell, crescentic outline, well-defined prominent tuberculate marginal carina in early growth becoming conspicuous angular tubercles later, an ornamented area with commarginal costellae that occasionally fade at late growth stages, with median groove; narrow oblique, diverging flank costae carrying numerous small regularly aligned tubercles, a smooth antecarinal space, and an escutcheon that is smooth or covered with transverse pustulose costellae.Myophorella Bayle (1878) differs from Promyophorella in having a triangular valve, a slightly orthogyrate to opisthogyrate beak, a conspicuously nodate median carina, strongly tuberculate flank costae, and by its large size.
Moerickella Echevarría, Damborenea, and Manceñido, 2021 (p. 26, fig. 13) can be easily distinguished by its flank ornamentation.It carries oblique costae, subparallel, anteroventrally sloping, and orthogonal to the antecarinal space, which bear well-developed elongated tubercles on their posterior segments.Fleming (1964Fleming ( , 1987) ) and Kelly (1995) considered Promyophorella as a junior synonym of Scaphogonia Crickmay, 1930. According to Francis (2000, p. 133), although Scaphogonia and Promyophorella are closely related, the subgenus Promyophorella should be used to describe those species of Myophorella that have a continuous set of highly aligned tuberculated costae across the flank.Echevarría et al. (2021) regarded Scaphogonia as a separate genus, related to Promyophorella, but differing by its flank ornamentation.
Orthotrigonia has V-shaped or L-shaped flank costae in early growth stages and steep, straight costae bifurcating anteriorly at later growth stages.For more details about the generic history of Promyophorella and comparison with other closely related genera, see Francis (2000, p. 133) and Echevarría et al. (2021, p. 31).
Based on the diagnostic features mentioned above, the present material can be assigned to Promyophorella with certainty.That genus is recorded here from the Lower Jurassic (Toarcian) rocks of Gebel Maghara, Egypt, for the first time.
Orthotrigonia gracilis (Kitchin, 1903) n. comb.can be easily distinguished by having seven arcuate costae in early growth stages, which pass over the marginal carina to cover the dorsal part of the area (Fig. 8.12).
Anterior and ventral margins strongly convex, meeting in continuous curve.Posterior margin narrowly rounded, meeting Escutcheon excavated, moderately wide, ornamented with small, rounded tubercles, forming transverse costellae.Ornamentation of flank consisting dorsally of seven arcuate costae with numerous fine, radially arranged riblets extending from their crests, where they form short spines.Arcuate costae cross over the marginal carina to cover dorsal part of area (Fig. 8.12, 8.15).Ventrally, flank costae extend radially to ventral margin and carry radially elongated tubercles.Costae separated by wide and smooth intercostal spaces.
Remarks.-The present specimen closely corresponds in outline and ornamentation of area and flank to the figures of Trigonia gracilis Kitchin (1903, pl. 9, fig. 7, 7a) from the Middle Jurassic (Callovian) of Kachchh, India.According to Fürsich and Heinze (1998, p. 164), the flank ornamentation of Orthotrigonia gracilis (Kitchin, 1903) n. comb.varies and they regarded these variations to be the result of intraspecific variation.The most diagnostic feature of the species appears to be that the arcuate flank costae of the early growth stages extend across the marginal carina to cover the dorsal part of the area (Fig. 8.12, 8.15).Other characteristic features are the low inflation (I/L = ?0.33) and small size (H = 10.8 mm).The most closely related species is O. jumarensis (Kitchin, 1903) n. comb.(Kitchin, 1903, p. 93, pl. 9, fig. 6) from the Middle Jurassic of Kachchh, India.Fürsich and Heinze (1998, p. 167) noted that the most diagnostic features of that species are the strongly tuberculated flank and a flat area covered with strong, but somewhat irregular commarginal growth rugae.Based on the description of Kitchin (1903, p. 95), the ornamentation of the flank of O. jumarensis n. comb.and O. gracilis n. comb. is very similar.Therefore, they are possibly synonymous.

Size of trigoniids from Gebel Maghara
In general, trigoniids were very minor elements of Jurassic macrobenthic communities with some notable exceptions such as the Oxfordian "Trigonia clavellata Beds" of Dorset, southern England (Fürsich, 1977) and the Kimmeridgian strata of Portugal where Myophorella lusitanica (Sharpe, 1850) and related species are quite common (e.g., Choffat, 1885Choffat, -1888; personal observation, F.T.F.).Trigoniids are also comparatively rare in the Jurassic succession of Gebel Maghara, (Abdelhady, 2014).Nearly all of the taxa from Gebel Maghara are of very small to small size, most specimens not exceeding 1-2 cm in height.This is quite unusual for the group, in which the average size of most taxa is several times larger.There are several potential explanations for the small size.(1) The individuals could be juveniles, stunted, or could represent small species, but there is no indication that the trigoniids are juveniles.In the Kimmeridgian part of the succession, they occur associated with normalsized brachiopods and other bivalves and there is no indication of size sorting (the fine-grained sediment pointing to low energy conditions).Moreover, the absence of any large adult specimens is difficult to explain.(2) Stunting, which cannot be ruled out with certainty, would imply adverse environmental conditions.However, normal-sized associated faunal elements and the moderate species diversity do not support such conditions.Stunting is, however, commonly known to affect benthic macrofauna in argillaceous substrates that experienced a certain deficiency of oxygen (e.g., Urlichs, 2011).Many of the infaunal trigoniids are found in silty marl and other infaunal bivalves, such as nuculids and the astartid Nicaniella, are commonly also smaller than elsewhere (personal observation, F.T.F.), therefore such substrates might not have been fully oxygenated.(3) With the present evidence we cannot exclude that the trigoniid fauna consists of small-sized taxa.In fact, some trigoniids do exhibit crowding of growth lines towards the ventral margin suggesting that they are adults, but other specimens are not well enough preserved to study this feature in detail.Regardless, whether these trigoniids are stunted to a certain degree or represent small species, their morphology does not just reflect juvenile but also adult features, and thus they should be described as taxa in their own right.

Conclusions
Based on newly collected material from the Jurassic strata of Gebel Maghara, Sinai, 14 species belonging to nine genera and two families of the bivalve order Trigoniida are systematically described, including the five new genera Magharitrigonia, Cotswoldella, Cornbrashella, Parorthotrigonia, Retetrigonia, and the new species Magharitrigonia asymmetrica.

Figure 1 .
Figure 1.(1) Locality map of Gebel Maghara.(2) Landsat image of the Gebel Maghara area showing positions of the three sections (numbered 1-3).(3) Geologic map (modified after Al Far, 1966; Hirsch, 1980) with positions of the investigated sections.(4) Satellite image of the Middle-Upper Jurassic succession; width of photograph corresponds to 1600 m. (5) Field photograph of the Middle to Upper Jurassic formations, exposed in the eastern saddle of the anticline of Gebel Maghara; thickness of Kehailia Formation is 250 m. (6) Jurassic subdivision and equivalent formations from older to younger (after Abdelhady and Fürsich, 2015a) with stratigraphic ranges of the trigoniid taxa.New genera are Magharitrigonia, Cotswoldella, Cornbrashella, Parorthotrigonia, and Retetrigonia.
from the upper Bathonian of France resembles Cotswoldella aff.C.

Table 1
5).Area of right valve wider and deeper than that of left valve, tripartite with well-developed tuberculated median carina and additional carina in dorsal half (Fig.4.1, 4.2), covered with widely spaced commarginal costellae (Fig.3.6).Flank inflated, triangular, occupying about two-thirds of shell surface, ornamented with 8-10 widely spaced commarginal costae (0.5-0.8 mm), asymmetrical in profile (steepest dorsally), terminating in node close to antecarinal sulcus and separated by very wide and smooth intercostal spaces (1.5-3.2 mm).Terminal nodes with distinct ventral extensions that are connected by thin radial riblet.Hinge of left valve wide, with large triangular cardinal tooth

Table 1 .
Measurements (in mm) of Magharitrigonia asymmetrica n. gen.n. sp.*Holotype; all other specimens are paratypes; see Figure 2 for measurement key.

Table 4 .
Parkinson, 1811espect to general outline and ornamentation of the area and flank, the Egyptian material corresponds very well to Cornbrashella pullus (J.de C. Sowerby, 1826) n. gen.n.comb.from the Bajocian-Bathonian of southern England.That species can be easily distinguished by its very small shell size and by having a wide area with well-developed radial costellae, a deep, ornamented escutcheon with transverse costellae, a small, smooth antecarinal sulcus, sharp opisthogyrate beaks, and strongly rounded anterior and ventral margins.Trigonia costataParkinson, 1811, of Francis (2000, p. 62,  pl. 1, figs.a-f; pl. 2, figs.a-f;pl. 3, figs.a-e)from the Middle Jurassic of England and Germany is somewhat similar to Cornbrashella pullus n. gen.n. comb.Although T. costata is a highly variable species, it differs from the present species in having a subtrigonal valve, an acute umbonal angle, a wide antecarinal sulcus, more numerous flank costae, a raised escutcheon (inset and depressed in C. pullus n. gen.n. comb.), and in being larger.
regarded T. langrunensisBigot, 1893  (p.287, pl. 2, fig.10)from the upper Bathonian of France as a synonym of C. pullus n. gen.n.comb.Despite the similarity between the two species, T. langrunensis differs in being larger, more elongated (longer than high) and in having numerous fine flank costae.For more details and comparison with other closely related Jurassic taxa, seeFrancis (2000, p. 95-98).Escutcheon small with transverse riblets.Hinge of right valve large with two striated cardinal teeth (3a, 3b), separated by a wide triangular socket (Fig.4.14).Anterior muscle scar small, subrounded, located close to anterodorsal margin.Flank weakly convex and ornamented with widely spaced, sharp, sub-straight costae topped with small tubercles (Fig.4.16).Costae separated by wide intercostal spaces and crossed by faint radial riblets forming a reticulated pattern (Fig.4.17).Remarks.-Theincompleterightvalve closely corresponds to similar specimens figured byCox (1952a, p. 111, pl. 12, figs.5-9) as Trigonia kheraensis Cox, 1952a, from the Middle Jurassic (Bathonian) of India.That species resembles the present material in having a very wide antecarinal sulcus and widely spaced and sub-straight commarginal flank costae, but differs in having a smooth area.The specimens figured by Mongin (1967, pl. 2, figs.16-19) as T. kheraensis from the Bathonian of Morocco have an ornamented area and escutcheon and resemble the Egyptian material more closely than the Indian species.

Table 12 .
Measurements (in mm) of Trigoniid gen.et sp.indet.See Figure 2 for measurement key.