RODENTIA. 



379 



tooth, and is interposed behind, between the 

 maxillary and the sphenoid. 



In the viscache and the chinchilla, the 

 bones of the nose are oval and elongated ; the 

 ascending branches of the intermaxillaries very 

 narrow at their origin ; but they enlarge as 

 they approach the frontal, as in the jerboas. 

 The maxillary, in both, forms the entire pre- 

 orbital ring; 'but in the viscache the vertical 

 portion of the arch is doubled posteriorly, as 

 in the helamys, by an ascending branch of the 

 jugal, by the lachrymal, and by the frontal 

 bones. At the bottom of the ring there is a 

 deep groove, almost entirely separated from it 

 by a vertical plate, as in the alactaga. In the 

 chinchilla, the jugal does not reach as far as 

 the lachrymal, and in the pre-orbital ring there 

 is only a very superficial furrow, with no ver- 

 tical plate of separation. In both species the 



Fig. 261. 



Base of the skull of the Chinchilla, 

 b, b, tympanic portion of temporal bone ; /, occi- 

 pital bone ; m, mastoid bone ; n, palate bone ; 

 o, the sphenoid ; p, the superior maxillary bone. 



palatine is very deeply notched, it articulates 

 with the maxillary, except externally, where a 

 point of the posterior sphenoid touches the 

 latter bone : moreover, on account of the en- 

 tire absence of the external wall of the ptery- 

 goid fossa, the palatine is found to occupy a 

 considerable space in the floor of the orbit, be- 

 tween the orbital alae and the maxillary : it 

 does not, however, mount upwards, as in the 

 kerodon, between the latter bone and the 

 frontal, to become connected with the lachry- 

 mal. The internal pterygoid apophysis is 

 largely connected with the tympanum. 



Bones of the carpus. Generally, in the 

 Rodentia, the os magnum is divided into two, 

 as it is in the monkeys ; in the porcupine this 

 is not the case, but there is a supernumerary 

 bone interposed between the os pisiforme and 

 the metacarpal bone of the fifth finger con- 

 nected with the os unciforme. 



The hare and some other Rodents have one 

 carpal bone more than the human subject ; it 

 is situated between the scaphoid, the trape- 

 zium, and the os magnum, of which last it 

 appears to be a dismemberment; but the 

 beaver, the marmot, the squirrel, the rats, and 

 the agoutis have, like the Carnivora, a single 

 bone representing the scaphoid. 



The supernumerary bone is as large as the 

 ordinary pisiform, and often much larger. 

 Sometimes, as in the jerboa and the marmot, 

 there are two supernumerary bones, so that, on 

 each side of the wrist, there is a bone of equal 

 size out of rank. 



In the capybara the scaphoid and the semi- 

 lunar bone are united without any supernu- 

 merary ossicle ; a small one, however, exists in 

 the Guinea-pig. The paca, the agouti, and 

 the capybara have the os magnum divided ; 

 these three animals possess, as the rudiment 

 of the thumb, a small bone situated upon the 

 trapezoid, with which it is articulated. 



In the marmot and the agouti this rudiment 

 is composed of three ossicles ; and there is, 

 moreover, an internal supernumerary bone. 



Fig. 262. 



Skeleton of the Hare (Lepus titnidus). 



