10(5 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



approaching Arvicola in stoutness, and especially in shortness of tail and 

 ears. Tail not one-half the length of the body alone, little if any longer 

 than the head, very stout, tapering to an obtuse point. Ears small — 

 about as in Evotomys — rounded, pilous both sides. Fore feet very large, 

 two-thirds to one-half as long as the hind, with large, little curved, fossorial 

 claws, longer than those of the hind feet ; the latter at most two-thirds as 

 long as the tail. Soles quadrituberculate only, densely furry to the tubercles. 

 Fur short, close, white beneath, as in Hesperomys. 



The remarkable mouse upon which the section was based is worthy of 

 subgeneric separation, at least, from the Hesperomys group of which leucopus 

 is typical. In its peculiar combination of characters, it stands quite alone 

 among North American species. Although unmistakably a true Murine, as 

 shown by the cranial and other fundamental characters, it nevertheless devi- 

 ates much from Mus and Hesperomys, and approaches the Arvicolines. Its 

 affinities with Evotomys are really close ; and it is through this genus that 

 the way for it into Arvicola proper is opened. In external form, indeed — the 

 stoutness of body, shortness of ears, and especially the shortness of tail — it 

 resembles Evotomys rutilus, for example, more than it does Hesperomys leu- 

 copus; while, at the same time, of its real affinity with the latter there can be 

 no question An interesting parallel can be drawn between Onychomys as 

 compared with Hesperomys, and Synaptomys as compared with Myoides. 

 Onycliomys has the skull and dentition of Hesperomys in a body externally 

 resembling Evotomys, while Synaptomys has the skull and dentition of Myoides 

 in a body externally resembling Evotomys* Onychomys offers another inter- 

 esting parallel. In the subgenus Pi/ymys as compared with Arvicola proper, 

 three external characters are shortness of tail, shortness of ears, and length 

 of fore feet and claws ; and therefore Onychomys stands in much the same 

 relation to Hesperomys that Pity my s docs to Arvicola. These analogies are 

 to us extremely interesting ; and the habits of Onychomys leucogastcr, when 

 fully known, will doubtless be found to offer some peculiarities corresponding 

 to the structural features. 



We are unable to point to any perfectly diagnostic characters of the 

 skull of this section as compared with that of Hesperomys. In size it about 

 equals the larger examples of 3. leucopus; it appears, however, somewhat 

 narrower behind, with less interorbital constriction, and broader and more 



* See further under head of Evotomys considered as a synthetic form. 



