ARTIODACTYLA. 
343 
while in the Bovidce the canines also have been retarded in development 
down to atrophy. 
In the genus Atichenia, as has been pointed out, the premolar teeth are 
two in number; in Poehrothermm of the Lower Miocene, they number four, 
the first and second of the normal Mammalian series being present. The 
first premolar is present in PoehrotJierium, Protolabis, Procatnelus, Pliauclie- 
nia, and Camelus. It is wanting in Auclienia and other Ruminantia. In the 
latter, it is present in the foetus, but soon disappears. In Auclienia^ accord¬ 
ing to Owen, it is retained for a somewhat longer time.* Thus retardation 
of the growth of this tooth is first seen in the latter genus so far as known, 
and is more pronounced in the other Ruminantia. The second premolar is 
present in PoehrotJierium, Protolabis, and Procamelus. It is absent in Pliau- 
cJienia, Camelus, and Auclienia. In the last two genera, it is a transitional 
character of immaturity, and w'-e may infer that this is also the case with 
PliaucJienia. It is thus evident that retardation, in the supply of nutritive 
material to this tooth, has caused its reduced size, and terminated the dura¬ 
tion of its existence. This has not occurred in the other lines of Ruminantia, 
where it remains as in PoehrotJierium. From these and many analogous 
cases, the general law may be deduced, that identical modifications of struct¬ 
ure, constituting evolution of types, have supervened on distinct lines of descent. 
It has been observed, as a remarkable fact, that North America should 
joresent us with the most complete history of the succession of genera which 
resulted in the Horse, and yet should have received this animal by impor¬ 
tation from Europe. Nevertheless, the more prominent genera of this series 
have been obtained in the European formations, especially AncJiitJierium 
and HippotJierium. But, as regards the Camelidce, the genera above de¬ 
scribed are exclusively North American, no w^ell-determined formf of this 
group having been found in any formation of the Palsearctic region up to 
the present time. Until such are discovered, there will be much ground 
for supposing that the Camels of the Old World were derived from Ameri¬ 
can ancestors, while the presence of the Llamas in the existing South 
* Odontography, p. 530. 
\ Merycoiherium, Bojanus (Nova Acta Acad. Loop. Car. Nat. Cur., vi), has not 
yet been distinguished from Camelus. 
