MURID^E— ARVICOLIN^;— 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, hut apparently an undescribed species 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 zygomatic 

 arch in 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 SynajHomys. 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 Muridce more strongly marked than the present, as the following 

 detailed descriptions will show. 



The most conspicuous and diagnostic, if not really the most important, 

 character is the sulcation of the upper incisors. This is a unique feature 

 among American Arvicolince, if not in the subfamily ; and, in the American 

 representatives of the whole family Muridcs, 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 Arvicolince 

 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 



