MURIDA—ARVICOLINAX—SYNAPTOMYS. 229 
the present investigation, while we were engaged upon Chilotus, and some- 
what short of material, we turned to this presumed Arvicola oregonus, moist- 
ened the skin, and carefully reversed it, when we found it was not Chilotus 
at all, but apparently an undescribed speciés of Evotomys. On cleaning 
the skull, of which we succeeded in securing, among other fragments, the 
lower jaw entire, the palate with the molars intact, half of one zy gomatic 
arch im situ, and the rostral portion with the incisors, we saw, to our sur- 
prise, that the cranial and dental characters were not in the least like those 
of Evotomys, but nearly identical with those of Myodes. It then only required 
a reference to Baird’s work for the recognition of Synaptomys. Shortly after 
this discovery, a series of seven perfectly prepared skins was sent us from 
Neosho Falls, Kans., accompanied by several nicely cleaned skulls. We are, 
therefore, able to define the form to our entire satisfaction. There is no genus 
of American Muridé more strongly marked than the present, as the following 
detailed descriptions will show. 
The most conspicuous and diagnostic, if net really the most important, 
character is the sulcation of the upper incisors. This is a unique feature 
among American Arvicolina, if not in the subfamily; and, in the American 
representatives of the whole family Muride, only recurs in Ochetodon of 
North, and Reithrodon of South, America. The groove runs near the outer 
edge of the face of the tooth (in Ochetodon and Reithrodon it is median). 
The incisors are short, broad for their length, and much curved; the general 
face is rabbeted down externally, so that when viewed in lateral profile the 
portions on either side of the groove appear parallel and one in front of the 
other. The incisors approach a character more fully developed in Myodes, of 
being essentially enamel tubes not completely filled up with dentine, thus 
calling to mind the condition of an unfinished quill-pen after the first oblique 
slice has been shaven off, and before the nib is finished. Their tips, instead 
of being straight and transverse, are generally nicked at the end of the groove. 
The inferior incisors are equally remarkable, not only in their own char- 
acters, but in the resulting modification of the under jaw. Exactly as in 
Myodes, their roots stop abruptly just in front, and a little to the inner side, 
of the root of the last lower molar. In all the other genera of Arvicoline 
we have examined, except Myodes and Cuniculus, the root of the under incisor 
runs past (outside) the root of the last under molar, and up the ramus of the 
jaw behind, to a varying distance toward the condyle itself. This passage of 
