1873.] 



Osteology of the Hyopotamidae. 



163 



few traces of this passage remain. From the tetradactyle DicJiodon, the 

 group of adaptive Selenodonts may be said to have split in two sub- 

 ordinate groups. In one of these, represented by the Hyomoschus, the 

 lateral digits are retained, and only the metatarsals become confluent, 

 while the two middle metacarpals continue to be free*. In the Tragulidos 

 the two middle digits coalesce, in both fore and hind limbs, into a complete 

 cannonbone, but the lateral digits are still retained in their whole length 

 as useless, nearly filiform appendages. The distal surface of the meta- 

 podium remains smooth ; the rumination is incomplete. 



In the other group, as the representative of which we may cite the 

 Gelocus, Aym., the lateral digits were soon lost, and the remaining two 

 middle digits have taken the entire distal surface of the carpus and 

 tarsus ; still they remain separate, perhaps through life, in some of the 

 Eocene Geloci whose remains I have seen from the phosphatic limestone 

 deposits in the south of France, near Cahors, in a locality called Caylux. 

 In this deposit the bones of Gelocus are found, together with large 

 Anoploiheria and Palceotheria, and even the completely ossified and not 

 epiphysed metatarsals are found entirely free. In the lowest Miocene of 

 Puy, however, we find a Gelocus whose metacarpals and metatarsals are 

 free only in the young, and coalesce in the adult ; but, even after their 

 coalescence, the distal end of the metapodium is smooth, and the articular 

 ridge is limited to the palmar side. In the somewhat newer (about the 

 upper part of the Lower Miocene) deposits of Allier, in Auvergne, we 

 meet at last with metatarsals and metacarpals entirely coalesced into a 

 complete cannonbone, and the articular ridge taking the whole distal 

 extremity of the metapodium. Small rudiments of the lateral digits 

 (second and fifth) still remain as styliform appendages on both sides of 

 the cannonbone, in the fore and hind limbs. 



Such true ruminant forms are exceedingly numerous in the Miocene of 

 Allier ; they are all hornless, and some retain seven molars in the lower 

 jaw, as in all ancient Selenodonts. In most, however, of these newer 

 Miocene forms the first premolar of the lower jaw is lost, and they 

 exhibit the same dental formula as the living Ruminantia, from which 

 they seem not to differ in any of the essential characters. These true 

 ruminant forms of the Lower Miocene may be considered to have reached 

 the elimination point of their reduction, and we shall consider them as 

 such. Thus the Selenodont Paridigitata, after branching off from the 

 common stock in the Lower Eocene, reach the utmost stage of re- 

 duction on the adaptive mode a little below the Middle Miocene ; this 

 we consider to be the fifth stage, or the culmination. 



The fifth stage, or the culmination point of the Paridigitata Selenodonta, 



* These middle metacarpals and metatarsals are enlarged and adapted to the whole 

 distal surface of the carpus and tarsus. 



VOL. XXT. P 



