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Show 562 SR. F. AMEGHINO ON THE [May 2, figure (7 b) demonstrates the origin of the similar characteristic molars in a considerable number of Ungulates-e.g., the Protero-theridse, Macrauchenidae, Meniscotheridae, Ehinoceridae, Titano-theridae, Palaeotheridae, &c. ; as well as in the long series of ruminant and selenodont Ungulata. In the ancient Pleuras-pidotheridae of France the form of the inferior molars of Proteodidelpliys is preserved almost without any change. The characteristic molar pattern of omnivorous Ungulata is the result of the atrophy of the median-anterior cusp ma and the median-posterior mp, or of their being intercalated in the same transverse line between the internal and external cusps of each lobe, ae, ai, and pe,pi. The lophodont pattern of the Tapir's molar is the result of the atrophy of the median-anterior cusp ma and of the union of the external cusps ae, pe with the corresponding internal ai, pi, by means of transverse crests. The origin of the molars of Pyrotheridae is the same, with the only difference that the median-posterior cusp mp is lengthened in a transverse direction, so as to form a sort of transverse heel (talon). The passage from the dentition of Pyrotherium to that of Dinotherium, and from this latter to that of Mastodon and of Elephas, is easily recognizable. In other Ungulata the median-posterior cusp mp became fused with the postero-external pe, in order to form a large external curved or crescentoid lobe, whilst the postero-internal pi approached the antero-internal ai; so that the two median cusps ma, mp became separated by three notches on the internal margin. The Horses (Equidae) are in this condition, as well as good number of Isotemnidae, the Homalodontotberidae, Leontinidae, and Tillodonts. The oldest known predecessor of the Horse series is Morphippus of the Upper Cretaceous. Fig. 8 shows its fifth right lower Fig. 8. Morphippus imbricatus: fifth right lower molar, slightly worn, superior aspect, twice nat. size.- Upper Cretaceous ; Patagonia. molar of a young individual, the six cusps being distinctly visible and partly independent. The successive changes leading to the Equidae are indicated by the same tooth of Morphippus in a worn condition (fig. 9), and by the corresponding tooth of tbe Upper Eocene Notohippus, represented in fig. 10 6, side by side with that of a recent Horse (fig. 10 a), so that the same elements with tbe identical fundamental disposition can be seen in them. |