Investigations apropos of the passage in Ovid to which we shall ultimately come have revealed that one kind of Latin genitive at least is still far from satisfactorily charted by authorities more eminent even than M'Turk. This is the genitive of material. More often than not grammarians and commentators do not distinguish this usage from the genitive of definition. So for instance at Ovid, Met. 3. 315 the phrase lactis alimenta is identified by Bomer ad loc. and by H. J. Roby (A grammar of the Latin language from Plautus to Suetonius (1896), ii. § 1302) as genitive of definition. This is correct as far as it goes, in the sense that lactis defines the alimenta; but the definition is what they were made of, their material. So again at Met. I. III flumina iam lactis, iam ftumina nectaris ibant (on which Bömer does not comment) the rivers consisted of milk and nectar. This passage, however, was classified by J. N. Madvig (A Latin grammar for the use of schools, trans. R. Woods, 4th ed. (Oxford, 1859), p. 249) under expressions involving number, measure or quantity. This is not unreasonable, but Madvig's examples illustrate the need for care in drawing the line. They include, for instance, aceruus frumenti (the genitive with aceruus is of course common) and uini tres amphorae. Now a heap may be thought of as containing its constituents, but it also consists o/them, which is not true of a vessel and its contents. Thus, though there is some semantic justification for including both these expressions, as Madvig did, under the ‘measure or quantity’ rubric, they are nevertheless grammatically distinct. The fluidity (!) of the borderlines in this area is evident also in flumina lactis, an undeniable genitive of material which is, however, clearly analogous to a not uncommon class of expressions such as riuus/fons aquae, lapidum imber, uolumina fumi, campus harenae, silicum uenae, ὔδατος ῥος, κρνη ὔδατος, ργρου πηγ, etc., in which the notion of quantity as well as composition is present.