RODENTIA—MURINAE—REITHRODON 447 
REITHRODON, Waterhouse. 
Reithrodon, Wareruouse, Pr. Zool. Soc. Lond. V, 1837, 30. 
Waaner, Suppl. Schreb. III, 1843, 545. 
Form murine. Ears and tail short, hairy. Upper incisors with a longitudinal channel along the anterior face. 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 furry hair. The tail is either longer ora 
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.) Theanterior 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, &e. 
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 Jetthrodon 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- 
