RODENTIA. 
121 
One species only is well detei'mined, the Chin- 
chilla of the furriers {Ch. lanigcra, Ben.)? cele- 
brated for the delicate fineness of its fur. It 
inhabits the Chilian and Peruvian Andes. 
Somewhat allied to the foregoing, is an- 
other small group of South American rodents, 
with also four rootless molars of equal size 
above and below, except in one instance 
{Abrocoma), where the inferior resemble those 
of an Arvicola; they are surrounded with 
enamel, and doubled, or indented deeply, on 
both sides. The antorbital foramen is very 
large. There are five toes to each foot, ex- 
cept in Abrocoma, which has only four anteriorly; and the general aspect is intermediate to that of the 
Chinchillas and Rats or Voles : the head, however, is arched. Four subdivisions have been distin- 
I guished. In 
The Abrocomes {Abrocoma, Waterh.), — ■ 
!' The ears are large, the claws very small, and the tail rather long and not tufted. The excessive 
j fineness of their fur probably exceeds that of any other animal. 
Two species were taken near Valparaiso by Mr. Darwin, A. Cuvieri and A. Bennettii, Waterh. 
I The Octodons {Octodon, Bennett; Dendrobius, Meyer), — 
li Have also large ears, and a long and tufted tail : their inferior molars resemble those of the following. 
The only known species (O. Cummingii, Ben.), is the Sciurus degus of Molina, D. degus, Meyer. It inhabits 
Chili, and is often seen traversing the branches of low underwood. 
I The Pcephagomes {PoepTiagomys, F. Cuv.), — 
;! Have narrow incisors, the auditory conch small, but distinct : claws adapted for burrowing. 
The only ascertained species (P. ater) 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 qualified for burrowing. 
There is a species in Brazil {Ct. braziliensis, Blainv.), and another near the Straits of Magellan (Ct. Magellani- 
cus, Ben.) 
A remarkable African rodent, which is in several respects allied to the last, is known as 
The Ctenodactyle {Ctenodactylus, Gray), — 
I The incisors of which are rounded ; there are but three molars, however, on each side of both jaws, 
suiTounded 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 
I 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 
ij great whiskers on the upper lip. 
|i But one species is known (C. Massonii, Gray), from North Africa; size of a Rat, with a short tail, and pale 
;i yellowish-brown fur, of very fine texture. 
I The foregoing arrangement of the extensive series of Rodentia is by no means reduced to 
j that simplicity which we conceive will ultimately be attained. Mr. Waterhouse, who has 
[ recently studied these animals very attentively, has succeeded in detecting several unexpected 
I 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 our entering into details ; so that it must suffice to state that, in general, the members 
i- 
