VERTEBRATA: MAMMALIA. 753 



present, does not differ in form from the other digits. The testes 

 pass periodically from the abdomen into a temporary scrotum, and 

 the placenta is discoidal and deciduate. 



The most characteristic point about the Rodents is to be 

 found in the structure of the incisors, which are adapted for 

 continuous gnawing hence the name of Rodentia. The in- 

 cisor teeth are commonly two in each jaw, and they grow from 

 persistent pulps, so that they continue to grow throughout the 

 life of the animal. They are large, long, and curved, forming 

 segments of a circle (fig. 438, A), and are covered anteriorly 

 by a plate of hard enamel. The back part of each incisor is 

 composed only of the comparatively soft dentine, so that when 

 the tooth is exposed to attrition, the soft dentine behind wears 

 away more rapidly than the hard enamel in front. The result 

 of this is that the crown of the tooth acquires by use a chisel- 

 like shape, bevelled away behind, and the enamel forms a per- 

 sistent cutting-edge. 



The gnawing action of the incisors is assisted by the articu- 

 lation of the lower jaw, the condyle of which is placed longi- 



Fig. 438. A, Side-view of the skull of Sciurus (Cynomys] Ludovicianus ; B, Molar 

 teeth of the upper jaw of the Beaver (Castor fiber). (After Giebel.) 



tudinally and not transversely, so that the jaw slides backwards 

 and forwards. The molars, consequently, have flat crowns 

 (fig. 438, B), the enamelled surfaces of which are always 

 arranged in transverse ridges, in opposition to the antero- 

 posterior movements of the jaw. The intestine is very long, 

 and the caecum voluminous (rarely wanting). The brain is 

 nearly smooth, and without convolutions. The orbits are not 

 separated from the temporal fossae, and the eyes are directed 

 laterally. The Rodents are almost all very small animals, and 

 they are mostly very prolific. They subsist principally, if not 

 entirely, upon vegetable matters, especially the harder parts of 



