180 Royal Society : — 



free*. In the Tragulidas 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 metapodium re- 

 mains smooth ; the rumination is incomplete. 



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

 the Oelacus, Aym., the lateral digits were soon lost, and the re- 

 maining 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 Gelaci 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 Oelacus are found, together with large Anoplotheria and Palceo- 

 theria ; and even the completely ossified and not epiphysed meta- 

 tarsals are found entirely free. In the lowest Miocene of Puy, 

 however, we find a Gelacus 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 meta- 

 carpals entirely coalesced into a complete cannonbone, and the 

 articular ridge taking the whole distal extremity of the meta- 

 podium. Small rudiments of the lateral digits (second and fifth) 

 still remain as styliform appendages on both sides of the cannon- 

 bone, in the fore and hind limbs. 



Such true ruminant forms are exceedingly numerous in the Mi- 

 ocene of Allier ; they are all hornless, and some retain seven molars 

 in the lower jaw, as in all ancient Selenodonts. In most, how- 

 ever, of these newer Miocene forms the first premolar of the lower 

 jaw is lost, and they exhibit the same dental formula as the living 

 Huminantia, 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 culmination 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 reduction 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 Sele- 

 nodonta, following the adaptive mode of reduction, means that the 

 reduction of the manus and pes was carried so far that it could 

 not proceed further ; this point was attained already in the Lower 

 Miocene. When once the metapodium was reduced to one bone, 

 and this one had taken the whole distal surface of the carpus and 

 tarsus, any further reduction or improvement was quite impossible. 

 Besides, the completely developed faculty of rumination gave these 



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

 whole distal surface of the carpus and tarsus. • 



