MAMMALIA. 



85 



Here the under-side of the toes, like the sole of the foot, is covered with minute 

 tubercles. 



Though in the form of the skull Abrocoma Cuvieri* agrees most nearly with 

 that of Octodon ; it differs in having the anterior portion narrower and rather 

 larger, compared to the part devoted to the protection of the brain ; the zygomatic 

 arch is shorter, the incisive foramina are longer, the body of the anterior sphenoid 

 is narrower, and the auditory bullae are larger. The principal differences observ- 

 able in the form of the lower jaw of Abrocoma, when compared with that of 

 Octodon, consists in the coronoid process being smaller, the condyloid narrower 

 from front to back ; the descending ramus more deeply emarginated posteriorly, 

 and the angle longer and more attenuated. 



In those characters in which the skull of Abrocoma departs from that of Octo- 

 don, it approaches nearer to Chinchilla. In the peculiar form and large size 

 of the ears, in the extreme softness of the fur, in the greater development of the 

 pads on the under side of the toes, and in the possession of only four toes to the 

 fore feet, there are other points of resemblance between Abrocoma and Chinchilla. 

 In the Chinchilla as well as in Octodon and Abrocoma, we find the toe corresponding 

 to the second (counting from the inner side) furnished with a broad hollow nail ;t 

 there are also stiff bristly hairs covering this nail as in the Oclodontidce. 



The extreme softness of the fur of the animals about to be described, sug- 

 gested for them the generic name of Abrocoma. The fur consists of hairs of two 

 lengths, and the longer hairs are so extremely slender that they might almost be 

 compared to the web of the spider. The specific names applied are those of the 

 distinguished naturalists who first made us acquainted with the two genera, Octodon 

 and Poephagomys. 



1. Abrocoma Bennettii. 



Plate XVIII. 

 Abrocoma Bennettii, Waterh., Proceedings of the Zoological Society of London, for February 1837, p. 31. 



A . corpore supra griseo, ad latera pallidiore et pallide cervino lavato, subtus albescenti- 

 cervino ; guld albescenti-grised ; pedibus sordide albis : auribus amplis, ad marginem 

 posticum rectis, extus ad bases vellere, sicut in corpore, obsitis: caudd corpore breviore, 

 ad basin crassiusculd, pilis brevibus incumbentibus vestitd. 



Description. — Form stout; ears large, with the posterior margin straight; fore 



* I have not had an opportunity of examining the skull of Abrocoma Bennettii. 



f This nail no doubt is used to cleanse the fur, and the bristly hairs may also assist in the operation; the two 

 small toes of the Kangaroo's hind foot are used for the same purpose. 



