MAMMALIA. 



77 



present the characters above pointed out as distinguishing the Arvicolida, 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 Chinchillidce, 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. 



EAST COAST OF SOUTH AMERICA. 



Maldonado. 



Mus decumanm. 



maurus. 



Musculus. 



tumidus. 



nasutus. 



obscurus. 



arenicola. 



— — bimaculatus. 



flavescens. 



Reithrodon typicus- 



See Proceedings of the Zoological Society for April 9th, 1839, p. 61. 



Mus Jacobise. 



Galapagoensis 



Coquimbo. 



Mus longipilis. 



Renggeri. 



— — Darwinii. 



