562 BR. r. AMEGHiwo ON THE [May 2, 



figure (7 h) demonstrates tlie origin of the similar characteristic 

 molars in a considerable number of Ungulates — e.g., the Protero- 

 theridaj, Macrauchonidse, Meniscotherkla?, Eliinoceridae, Titano- 

 theridse, Palaeotherid*, &c. ; as well as in the long series of 

 ruminant and selenodont Ungulata. In the ancient Pleuras- 

 pidotheridiB of France the form of the inferior molars of 

 Proteodiddpliys 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 joe,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, p>i, by means of transverse crests. The origin of the 

 molars of Pyrotheridge 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 Pijrothermm to that of Dinotlierium, 

 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 ^j^, in order to form a large external 

 curved or crescentoid lobe, whilst the postero-intei-nal pi approached 

 the autero-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 

 Isotemnidse, the Homalodoiitotherid£e, Leontiuida?, 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. 



nrul 



Morphippus imhricatus: 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 

 Equida; are indicated by the same tooth of Morphippus in a worn 

 condition (fig. 9), and by the corresponding tooth of the Upper 

 Eocene NotoUppus, represented in fig. 10 J-, side by side with that 

 of a recent Horse (fig. 10 a), so that the same elements with the 

 identical fundamental disposition can be seen in them. 



