RODENTIA. 



121 



One species only is well determined, tlie Chin- 

 chilla of the furriers {Ch. lanigei-a. Ban.), cele- 

 brated for the delicate fineness of its fur. It 

 inhabits the Chilian and Pernvian Andes. 



Somewliat allied to the foregoing, is an- 

 other small group of South American rodents, 

 with also four rootless molars of equal size 

 r a))ove and below, except in one instance 

 ^= {Al/rocoma),\\\\QXQ, the inferior resemblethose 

 —^ of an Arvicola ; they are surrounded with 

 enamel, and doubled, or indented deeply, on 

 both sides. The antorbital foramen is very 

 Fiii.-i3.-TiieChiiichiiu. large. There are five toes to each foot, ex- 



ce])t in Ahrocoma, \\\\\ -Xi hrs only four anteriorly; and the general aspect is intermediate to that of the 

 Chinchillas and Rats or Voles: the head, however, is arched. I'our subdivisions have been distiu- 

 guislied. In 



The Abrocomes {Ahrocoma, AVatcrli.), — 



The ears are large, the claws very small, and the tail rather long and not tufted. The excessive 

 fineness of tlieir fur probably exceeds that of any other animal. 

 Two .species were taken near Valparaiso by Mr. Darwin, A. Ciivicri and A. BomcilU, Waterh. 



The Octodons {Octodon, Bennett; Dendrohius, Meyer), — 

 Have also large ears, and a long and tufted tail : their inferior molars resemble those of the following. 



The only known species (O. CummivgU, Ben.), is the Sciurus dcjus of IMolina, Z>. dajns, Meyer. It inhabits 

 Chili, and is often seen traversing; the branches of low underwood. 



The PfEPHAGOMES {Poej)hogomijs, F. Cuv.),— - 



Have narrow incisors, the auditory conch small, but distinct : claws adapted for burrowing. 



The only ascertained species {P. aicr) inhabits Chili. 



Finally, 



The Ctenomyds {Ctenomys, Ben.)— 



Are distinguished by the great breadth of their incisors, by the smallness of their ears, their rather 

 short tail, and stout claws, well qualitied for Lmrrowing. 



Tliere i^ a species in Brazil (Ct. hvazUiensis, Blainv.), and another near the Straits of Magellan (Ct. Magellani- 

 cus, Ben.) 



A remarkable African rodent, which is iu several respects allied to the last, is known as 

 The CTENonACTYLE (Cfenodacfglus, Gray), — 

 The incisors of which are rounded ; there are but three molars, however, on each side of both jaws, 

 surrounded with enamel, the upper with one deep indentation externally, the lower indented on both 

 sides. The feet have each four toes, with the rudiment of a thumb on the anterior ; and the hinder 

 especially are furnished with stiff brush-like bristles, -which curve over the toes (a structure which is 

 also seen in the last preceding subdivisions). The general aspect resembles that of the Chinchilla 

 group, to which the structure of the lower jaw bears also some resemblance ; and there are similar 

 great whiskers on the upper lip. 



But one species is known (C. Ulassoiiii, Gray), from North Africa; size of a Ilat, with a short tail, and pale 

 yellowish-brown fnr, of very fine texture. 



The fore2;oing arrangement of the extensive series of Rodentia is by no means reduced to 

 that simplicity which we conceive will ultimately he attained. Mr. Waterhouse, ■who has 

 recently studied these animals very attentively, has succeeded in detecting several unexpected 

 affinities which tend to this result: and he finds that the most useful or least variable charac- 

 ters, indicative of the mutual relations of the several genera, are derivable from the configura- 

 tion of the cranium, and especially that of the lower jaw. The space allotted in this work 

 forbids oiu- entering into details ; so that it must suffice to state that, in general, the members 



