The species of the genus Hesperomys, which depart most from the type—whose dentition is least like figs. 5, a, and 5, b, Plate 33., or 6, a, and 6, b, of the same Plate—recede still farther from the genus Mus, and approach more nearly (as regards the dentition) to the Arvicolidæ. Among the species here described I may mention as examples, M. griseo-flavus, M. zanthopygus, and M. Darwinii;—see the molar teeth figured in Plate 34. figs. 15, 16, and 17,—and among the North American species, those constituting the genus Neotoma. The latter make by far the nearest approach to the Arvicolidæ of any which have yet come under my observation, not only in the dentition, but in the form of the skull and the large size of the coronoid process of the lower jaw; there is, nevertheless, a tolerably well marked line of distinction between the crania of the Arvicolidæ and Neotoma.

The skulls of the animals belonging to the genera Castor, Ondatra, Arvicola, Spalax, and Geomys, which constitute the principal groups of the family Arvicolidæ, when compared with those of the family Muridæ, present, among others, the following distinctive characters.

The temporal fossæ are always much contracted posteriorly, by the great anterior and lateral development of the temporal bones; the plane of the intermolar portion of the palate is below the level of the anterior portion; the coronoid process of the lower jaw is very large, the articular portion of the condyloid process is proportionately broad; the descending ramus, or posterior coronoid process, is so situated that its upper portion terminates considerably above the level of the crowns of the molars; this same process is generally[[30]] directed outwards from the plane of the horizontal ramus. The incisor teeth of the Arvicolidæ differ from those of the Muridæ in being proportionately broader and less deep from front to back—they are not laterally compressed as in Mus. The molar teeth are rootless,[[31]] and the folds of enamel are the same throughout the whole length of the tooth; whereas in Mus they enter less and less deeply into the body of the tooth as we recede from the crown, and towards the base of the visible portion (the tooth being in its socket) the indentations of the enamel are obliterated.

Now in the species of Hesperomys, the molar teeth are always rooted, and in the form of the skull and the lower jaw they agree with the Muridæ, and do not present the characters above pointed out as distinguishing the Arvicolidæ, and as regards the cranium and lower jaw, it is only in the genus Neotoma that any approach is evinced.

Of the various groups of the order Rodentia found in South America, the Sciuridæ, so far as I am aware, are chiefly confined to the more northern parts, and do not occur in the most southern; the Myoxidæ, Gerboidæ, and Arvicolidæ are wanting. The species of the family Muridæ belong to different sections to those of the Old World. Of the Leporidæ I am acquainted only with one well established 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 Hystricidæ, the genera, Echimys, Dasyprocta, Cælogenys and Myopotamus, together with the Octodontidæ and Chinchillidæ, 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 Caviidæ, 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[[32]] the transitions between one form and the other are found.

The South American Muridæ, 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 Jacobiæ.

—— Galapagoensis.