RODENTIA MURINAE HESPEROMYS. 455 



OXYMICTERUS, Waterhouse. 



Oxymiderus, WATEHHOUSE, Proc. Zool. Soc. V, 1837, 30. 



WAGNER, Suppl. Schreb. Siiug. Ill, 1843, 513. 

 BURMEISTER, Tliicre Bras. I, 1854, 182. 



Form, Lemming-like. Eyes and whiskers small ; tail and feet short. Claws very long and sharp ; thumb with a distinct 

 claw. Muzzle prolonged into a trumpet-shaped tube. Central molar in either jaw with only one indentation on each 

 side ; the very small posterior molar with none at all. 



Where Habrothrix represents the Arvicolas, according to Burmeister, the species of Oxymic- 

 terus bear the same relation to the Lemmings, as far as the stout body, short tail and feet, long 

 claws, and the distinct claw on the thumb of the fore foot extend, instead of a broad nail, as in 

 Calomys and Habrothrix. The muzzle is very large and pointed, more so than in Habrothrix; 

 but like it, the fur is short, with the coarse hairs projecting but slightly ; eyes small and the 

 whiskers feeble. The ears are well furred. The soles are naked from the heel. The edge of 

 the intermaxillary is prolonged into a tube completed by the elongated nasal bones widening 

 at the end, somewhat like a trumpet. The incisive foramina extend back between the molars ; 

 the ante-orbital foramen is small. The upper edges of the orbit are much rounded, the fore 

 head broader between the orbits than in Calomys, the nasal bones extending much further back. 

 The molars have short wide folds when worn, somewhat similar to those of Arvicola, without 

 any enamel islands. 



A careful comparison of the species of the Sigmodont group of North and South America is 

 quite sufficient to show that, with the same general characters, the minor conditions vary 

 throughout. I have not had, as yet, the opportunity of examining more than one South American 

 species ; but judging from this, and from the very accurate figures and descriptions of Waterhouse, 

 Wagner, Burmeister, and others, I consider it entirely probable that a very critical and extended 

 investigation will show that even the genera, much more the sub-genera, are entirely different. 



A striking feature of the North American vesper mice, to anglicise Wagner's name, is their 

 diminutive size compared with the South American. Many species of the latter are fully equal 

 to the rats, or even larger, some of them, as Holochilus, with still larger teeth. Scarcely one 

 of our species exceeds four inches to the root of the tail in the flesh, while most are the size of 

 the common house mouse or less. The South American Reithrodons have a body six inches in 

 length, so stout and full, and the head so large and much arched, that one species has been called 

 R. cuniculoides, rabbit-like. The tail, also, does not exceed half the length of the body. Our 

 species, on the contrary, are the smallest of our mice, scarcely more than half the size of the 

 house mouse, which they otherwise closely resemble in shape and proportions. The tail is as long 

 as the body alone, or else longer than the head and body together. The shape and character of 

 the skull are quite different. 



The only one of the South American subdivisions of the genus Hesperomys to which our North 

 American animals eVen approximate is Calomys. Even here the South American species are 

 mostly of large size, only a few as diminutive as the largest of ours. None of the divisions of 

 Habrothrix, Scapteromys, Oxymicterus, or Phyllotis, not to mention Holochilus, are represented 

 in any North American species. 



It then becomes necessary to seek for some other arrangement for the North American vesper 

 mice, and this is readily indicated by an examination of the species now known to naturalists 

 in which sub-generic distinctions, as well denned as those of the South American, can easily be 

 found. I have satisfied myself of the existence of four groups, readily distinguishable by 



