RODENTIA MURINAE RE1THRODON. 447 



REITHRODON, Waterhous^. 



Rtilltrodan, WATEBHOCSE, Pr. Zool. Soc. Lond. V, 1837, 30. 

 WAGNER, Suppl. Schreb. Ill, 1843, 545. 



Form murine. Ears and tail short, hairy. Upper incisors with a longitudinal channel along the anterior faco. Other 

 characters much as in Hesperomys. 



Although the general appearance of this genus is much like that of Hesperomys, yet, as Wag 

 ner pertinently remarks, in a group embracing so large a number of species, and resembling 

 each other so closely as Hesperomys, the fact that Reithrodon has the sharply defined character 

 of grooved incisors is legitimate ground for separating the two. There are, however, other 

 characteristics of value which will be adverted to hereafter. 



But three species have been hitherto described from South America, all from its extreme 

 southern portion ; and it is for this reason a matter of some surprise to find the genus repre 

 sented in the United States as first announced by Major Leconte. Ours are, however, con 

 siderably smaller than the South American, and differ in several other points, as will be indi 

 cated hereafter. 



The South American species are said to resemble small rabbits in their general appearance. 

 The North American are more like slender house mice. The body is depressed, the limbs 

 short. The head is considerably shorter, broader, and more arched than in Hesperomys ; the 

 ears shorter, lower, and quite densely covered with lurry hair. The tail is either longer or a 

 little shorter than the body, and coated with hair. The thumb is rudimentary, with a blunt 

 nail ; the hind part of the soles, at the heel, hairy. 



The grooving of the upper incisors is much like that of Jaculus, only deeper, (the incisor con 

 siderably more curved.) The anterior edge is rabbetted down on its entire outer half somewhat 

 below the level of the inner portion, and the edges rounded ; both portions of the anterior face, 

 as seen from in front, are symmetrical, and strongly convex transversely ; viewed laterally, the 

 outlines are distinctly visible, parallel to each other, and one behind the other. The molars, in 

 shape and indentations, are much as in the North American Hesperomys (Calomys). The skull 

 is short, and the cranium swollen and strongly convex in all directions ; the upper edge of the 

 orbit sharp and continuous, with a low ridge on the cranium. In the great width and con 

 vexity of the cranium there is much difference from Calomys. Other differences are seen in the 

 backward prolongation of the incisive foramina to the line of the first molars ; the larger size of 

 the palato-maxillary foramina ; the greater divergence of the axes of the auditory bullae ; the 

 greater posterior extension of the palate, &c. 



The most striking difference in the lower jaw is seen in the abrupt twisting inward of the 

 lower half of the descending ramus. This inferior half is quite horizontal in position; and 

 when the jaw is viewed laterally, there seems to be as much of the ramus as in Calomys, with 

 out taking into account the hidden internal portion, which in the latter is very much less in 

 extent. 



The general appearance of the North American species of Reithrodon is intermediate between 

 that of the common domestic mouse and the white-footed mice. The tail is nearly as long, or 

 longer, than the body ; the ears smaller than in the Hesperomys leucopus, more buried up in the 

 fur ; the membrane thicker and covered with longer hairs. The antitragus is very much de- 



