84 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



In the Octodontidce the skull is rather short, the inter- orbital space is broad ; 

 the ant-orbital passage is large ; the zygomatic arch is thrown out horizontally 

 from the plane of the palate ; the malar bone is broad and somewhat compressed, 

 and throws up a small post-orbital process ; the glenoid cavity of the temporal 

 bone is narrow ; the palate is contracted, and deeply notched posteriorly, the 

 portion which lies between the molar teeth descends below the level of the anterior 

 portion ; the incisive foramina are wide : the body of the anterior and posterior 

 sphenoids is very narrow, and the foramina on either side of them are large : the 

 occipital condyles are very narrow, widely separated, and the articular surface 

 is nearly vertical.* The descending ramus of the lower jaw springs from the 

 outer side of the alveolar portion, and terminates in a point, more or less acute. 



The incisors of the upper and lower jaws are of the same width : the molars 



are ti|, rootless. 



In external characters the species of the present group vary considerably. 

 The toes are 5 1 5 or 4 1 5, The claws of the hind feet are covered by strong, curved 



bristly hairs. 



The principal points of distinction in the external characters of the four 

 genera under consideration, may be thus expressed. 



t TOES 5\5. 



A. Fore feet formed for burrowing — strong and armed with large claws; tail short. 



a. Ears minute, incisors very broad ....... 



b. Ears small, incisors broad ......... 



B. Fore feet weak ; claws small ; incisors narrow ; ears large. 



Ctenomys. 

 Poephagomys. 



a. Tail with the apical portion furnished with long hair , Octodon, 



ft TOES 4|5. 



■ 



b. Tail furnished throughout with short adpressed hairs . Abrocoma. 



It is not only in the comparatively small size and weakness of the fore feet 

 that Abrocoma approaches more nearly to Octodon ;. but it agrees in having the 

 soles, both of the fore and hind feet (which are devoid of hair), covered with minute 

 round fleshy tubercles (see the under side of the tarsus figured in Plate 28.) 



In Octodon, however, the toes have on their under side transverse incisions, 

 as the Muridce, and many other Rodents ; a character not found in Abrocoma. 





* There is a wide difference between the present animals and the Arvicolidce in the form of the occipital 

 condyles : the same difference is also observable between Echimys and Mus. The Octodontidce in fact have the 

 same form of condyles as the Chinchillas and Cavies. In this and many other characters the last mentioned 

 animals evince an affinity to the Leporidce. 



