290 EXTINCT GENERA OF CAMELS 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 Poehrotherium, 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 is 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, Escliatia, and Holo- 

 meniscus. The last two are Pliocene and American : the teeth 

 are much reduced. 



C. PEOORA. 



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.^ Tlie 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 lining 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 



' Unless Protoceras (see p. 284) was famished witli horns. 



