Collections of fossils from the Lower Palaeozoic rocks of the Arctic usually contain species of Streptelasma. These, with other forms, have often been considered indicative of horizon. Now it is unfortunately true that none of the original descriptions or figures of species described from the Arctic, or of those relevant species long known in America, are adequate for strict identification. It is a matter for no surprise, therefore, that many false identifications have been made.

In the southern belt of the Ballantrae serpentine, stretching south-west from Millenderdale, there occur many exposures of altered and more or less recrystallized dolerite and gabbro. Some of these rocks have afforded considerable difficulty to previous observers and they have been variously interpreted. Thus Dr. G. W. Tyrrell, in an interesting paper published twenty-five years ago (Trans. Geol. Soc. Glas., xiii, 283), regarded some of the rocks on Littleton Hill as “relics of an older mass of dolerite or gabbro” that had “become enveloped by the later serpentine”. But this expression of opinion does not appear to have been founded upon conviction, since quite recently Tyrrell states (Proc. Geol. Assoc, xliv, 59) “contact metamorphism presumably by the enveloping serpentine seems to have been developed in rocks on Littleton Hill which may originally have been spilites and diabase porphyrites”. At the time of writing a previous paper (GEOL. MAG., LXIX, 1932, 107) I hadalready inferred that the metamorphic rocks seen on Littleton Hill must have been derived from dolerites and gabbros, but the nature of their association with the serpentine was then merely a matter of surmise. In continuing field work in the province, therefore, I keptin mind the requirement to determine, if possible, the true geological relationships of these rocks, and to that end I have re-investigated carefully the whole southern outcrop of the serpentine and its accompanying metamorphic masses. A short account of the results of the field and laboratory work is given below. Firstly, however, I take leave to acknowledge the very helpful character of Mr. A. G. Macgregor's researches on felspar clouding, “a characteristic effect that is often produced in the fresh plagioclase felspars of igneous rocks by thermal metamorphism” (see Min. Mag., xxii, 524).

The name of this species was proposed by Schlotheim (1832, 65, No. 88), but the history of the form really goes back to Linnaeus. The earlier writers on the Scandinavian Palaeozoic took the name Anomia plicatella Linn. (1758, 702) to cover a great number of different ribbed brachiopods, and referred in their synonymies to the form figured in fig. 5, of tab. v, of Linnaeus's Museum Tessinianum (1753), a work published some years before the adoption of binomial nomenclature. When different authors began to subdivide this group, the name plicatella came to be restricted to a species referable to the genus Spirifer, and though some authors, Wahlenberg (1821, 67) and Dalman (1828, 140, Tab. vi, fig. 2), continued to use the name plicatella in a wide sense, so as to include the species under consideration, the latter seems to have come within the range of forms described by Schlotheim (1813, 36) as Terebratulithes lacunosus. This author apparently had the intention of separating a group of ribbed Silurian brachiopods, under the name of Terebratula borealis, for in the catalogue of his collection (Schlotheim, 1832, 65, No. 88) six specimens from the “Uebergangskalkstein” of Sweden are entered under this title. The term, however, remained a nomen nudum until von Buch (1834) gave an adequate description, and it is to this author that the species should correctly be ascribed. He chose as type a specimen from the Encriniten-Übergangskalkstein of Gotland, figured in an earlier work by Schlotheim (1822, 68, Taf. xx, fig. 6a—c) as Anomia Terebrat. lacunosus.