290 EXTINCT GENERA OF CAMEES CHAP. 
there are more points of likeness to the Lama, in P. angustidens 
to the Camel. In both, the orbits are completely encircled by 
bone. The nasals are much shortened. The odontoid process of 
the axis is still more concave than in Poebrotheriwm, but not spout- 
like as in existing forms. This fact shows that the spout-like 
character of the Camels’ odontoid process is not a point of affinity 
to other Artiodactyles—in fact the occurrence of the same form of 
- 
odontoid process in Perissodactyles is enough proof of this. We. 
must come to the conclusion that the form is adaptive in all cases. 
If we were not obliged on palaeontological evidence to come to this 
conclusion, the structure in question is just one which would be 
fastened upon as evidence of genetic affinity; for it 1s a resem- 
blance in a small though distinctive point of structure having no 
obvious relation to utility. The metacarpals and metatarsals 
have coalesced to form the cannon bones, though a rudiment of 
one metacarpal seems to remain. ‘The genera referred to appear 
to be on the direct line of descent of the modern representatives 
of the family. But there are other forms which are offshoots of 
the main stem. Such are Homocamelus, Eschatia, and Holo- 
meniscus. The last two are Pliocene and American: the teeth 
are much reduced. 
C. PECORA. 
The Pecora are a group which possess so many characters in 
common that it is not an easy task further to subdivide them. 
In all there are but two functional digits on the feet, and the 
metacarpals and metatarsals of these are fused. There are no 
upper incisors, and canines in the upper jaw are not universal, 
and generally small. Horns are confined to this group of the 
Selenodontia.* The premolar teeth are of a simpler form than 
the molars. The stomach has four chambers, of which two 
may be regarded as belonging to its’ cardiac half and two to 
the pyloric. The former are, in the first place, a large paunch 
or rumen, followed by a smaller reticulum, so called on account 
of the network arrangement of the folds of its liming membrane. 
Connected with the latter, and constituting the first part of the 
pyloric half of the stomach, is the psalterium or “ manyplies,” so 
called on account of the longitudinal folds, like the leaves of a 
1 Unless Protoceras (see p. 284) was furnished with horns. 
