106 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. Hars 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 gars, and especially the shortness of tail—it 
resembles Evotomys rutilus, for éxample, 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 Ayoides. 
Onychomys has the skull and dentition of Hesperomys in a body externally 
resembling Evotomys, while Synaptomys has the skull and dentition of Myotdes 
in a body externally resembling Lvotomys.* —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 Pitymys does to Arvicola. hese analogies are 
to us extremely interesting; and the habits of Onychomys leucogaster, 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 H. deucopus; it appears, however, somewhat 
narrower behind, with less interorbital constriction, and broader and more 
*See further under head of Lvotomys considered as a synthetic form. 
