MAMMALIA. 
77 
present the characters above pointed out as distinguishing the Arvicolidce, and as 
regards the cranium and lower jaw, it is only in the genus Neotoma that any ap- 
proach is evinced. 
Of the various groups of the order Rodentia found in South America, the 
Sciuridce, so far as I am aware, are chiefly confined to the more northern parts, and 
do not occur in the most southern ; the Myoxidce, Gerboidce, and Arvicolidce are 
wanting. The species of the family Muridce belong to different sections to those 
of the Old World. Of the Leporidce I am acquainted only with one well es- 
tablished species — the Lepus Braziliensis, which however is not found “in tota 
America Australi,” as Fischer says, there being no Hare yet found in the more 
southern parts, where the Cavies and Chinchillas appear to take their place. The 
remaining South American Rodents — certain species of Hystricidce, the genera, 
Echimys, Dasyprocta, Ccelogenys and Myopotamus, together with the Octodontidce 
and Chine hillidce, all possess a peculiar form of skull and of the lower jaw, (more 
or less approaching to figs. 1, Plate 33, and figs. 23, Plate 34.) which I have 
described in the “ Magazine of Natural History,” for February 1839, and which is 
rarely found in the North American, or Old World Rodents. In enumerating the 
above groups, I omitted the Caviidce, because in the form of the lower jaw they 
differ somewhat from the rest — they possess, in fact, a form of lower-jaw peculiar 
to themselves; but in the Chinchillas* the transitions between one form and the 
other are found. 
The South American Muridce , which form the chief part of Mr. Darwin’s 
collection, were none of them procured further north than latitude 30°, with 
the exception of those from the Galapagos Archipelago. The species occur at 
the following localities. 
WEST COAST OF SOUTH AMERICA. 
Galapagos Archipelago. 
Mus Jacobite. 
Galapagoensis. 
Coquimbo. 
Mus longipilis. 
Renggeri. 
Darwinii. 
EAST COAST OF SOUTH AMERICA. 
Maldonado. 
Mus decumanus. 
maurus. 
Musculus. 
tumidus. 
nasutus. 
obscurus. 
arenicola. 
bimaculatus. 
■ flavescens. 
Reithrodon typicus. 
See Proceedings of the Zoological Society for April 9th, 1839, p. 61. 
