OBSERVATIONS ON THE RODENTU. 597 



temporal bone ; behind this cavity the temporal bone is di- 

 lated, and forms a large and deep hollow, which apparently 

 receives the broad condyle of the lower jaw when it is drawn 

 back. The ant-orbital outlet is larger than in other Arvico- 

 lid(B. In the skull before me the suture between the nasal 

 bones is completely obliterated in front. 1 But the most re- 

 markable character in this skull is the form of the occiput (d), 

 which is enormously large, and instead of being as usual ver- 

 tical, in its oblique direction 2 reminds us of the occiput in 

 the Cetacea, seals, and some other aquatic mammals. 



The upper and lower margins of the occipital opening, in 

 nearly all the skulls of Arvicolida which I have examined, 

 are situated in the same vertical line, that is to say, a straight 

 line touching the upper and lower boundaries of the foramen 

 magnum is at right angles with the plane of the skull. Upon 

 first observing this character in the beaver 3 and Ondatra, I 

 imagined that having connexion with the position of the head, 

 it might be attributed to the aquatic habits of these animals, 

 being aware that the same character was found also in the 

 seals, whales, and some other aquatic mammals ; I perceived 

 however, upon further examination, that in the Coypus 

 [Myopotamus Coypus) and the Capibara {Hydrochceres Ca- 

 pibara), two other aquatic rodents, the upper and lower 

 boundaries of the occipital foramen did not differ in this 

 respect from the terrestrial species; whereas, on the other 

 hand, in Spalax, the lower boundary of this opening projects 

 beyond the upper, and forms an angle of about 93° ; it is re- 

 markable therefore, that the great angle formed by the boun- 

 daries of the occiput occurs in all the Arvicolidce. In Arvi- 

 cola agrestis and Geomys umbrinus ? the upper margin of the 

 foramen magnum projects slightly beyond the lower, yet in 

 these animals the upper and lower boundaries of this open- 

 ing approach more nearly to a vertical line than usual. 



The form of the lower jaw in the animals of the present 

 family affords an easy character by which they may be dis- 

 tinguished from other rodents, at least from those whose skulls 

 I have examined : the peculiar position of the angle of the 

 jaw, however, is not striking unless the jaw be placed in its 

 natural position, that is, so that all the molars meet those of 



1 In the skull of a species of Geomys I find the nasal bones anchylosed 

 in like manner. 



9 In the crania of Arvicolce and Ondatra, there is a slight approach evinced 

 to this form of occiput, and in Geomys the occiput slopes forwards in a 

 considerable degree. 



3 1 ought to say the adult beaver, for in the young animal the upper 

 boundary of the foramen magnum projects over the lower. 



