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106 



MONOGEArHS OF NORTH AMERICAN RODENTIA. 



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approaching Arvicola in stoutnc-s, and especially in shortness of tail and 

 cars. 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 th' ids to one-half as long as the hind, with large, little curved, fossorial 

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

 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 docs Hesperomys leu- 

 copus; wiiile, 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 Synaptoniys as compared witii Myoides. 

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

 resembling Evotomys, while Synaptomys lias the skull and dentition oi Myoklef 

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

 esting parallel. In the subgenus Fi/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 Onychotnys stands in much the same 

 relation to Hesperomys that Vitymys does to Arvicola. These analogies are 

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

 I'ully known, will doubtless be found to offer some peculiarities corresponding 

 to the structural features. 



We arc 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. leucopus ; it appears, however, somewhat 

 uarrow<!r beiiind, witii less interorbital constriction, and broatler and more 



