MURIDJ3— SIGMODONTES— ONYGHOMYS. 107 



obtuse rostrum. The margins of the orbits are not beaded as in Oryzomys. 

 The descending process of the lower jaw is, perhaps, not quite so flat and 

 quadrate, being a trifle more bent and angular, slightly foreshadowing the 

 hamular process of Arvicolince ; and, likewise, the coronoid is longer, rising 

 high above the condyle. To our view, the molars present unmistakably a 

 slight deviation from Hesperomys toward Arvicolince. in being somewhat pris- 

 matic ; that is, with sharp salient and reentrant angles; the borders of the 

 molar series being serrate, rather than crenate as in Hesperomys. Never- 

 theless, no generic dental characters of Hesperomys are here violated. 



On casually picking up a specimen of Onychomys leucogaster, one 

 might be forgiven for supposing the tail to be broken off — this member is so 

 short and "stumpy". The base is very thick, and the tapering to an obtuse 

 point is rapid. The tail is well clothed with hairs, as usual ; but these only 

 furnish an apology for a terminal pencil. Both the absolute and relative 

 length of the tail is shown in the table below. The large fore feet, armed 

 with remarkably long and little curved claws, point to fossorial powers, and 

 possibly to subterranean habits, not shared by any other Hesperomys. Next 

 in importance, after the relative sizes of the fore and hind members (which 

 latter are unusually short), must be ranked the peculiar condition of the soles, 

 in which the two posterior of the six tubercles usually seen both in Hespero- 

 mys and in Arvicola have disappeared, or at least cannot be detected beneath 

 the fur that densely covers the metatarsus, leaving only four tubercles at the 

 bases of the toes. The middle, finger is the longest of all; the fourth next; 

 the first is rudimentary ; the fifth is between the second and first ; the second 

 between the fifth and fourth. The inner toe is very short; the outer inter- 

 mediate between this and the other three, which are all of about the same 

 length. The ears, as we have said, are, in the type of the subgenus, much 

 smaller than in Hesperomys proper, and very much as in Evotomys ; they are 

 closely pilous both sides. 



The fur is very compact and lustrous, with few long hairs ; the whiskers 

 are numerous and very long; the eyes are larger than in Arvicolines, but have 

 not the size and prominence of those of the true Murines. The upper lip is 

 cleft to the nasal papillae, and the entire muffle, except these protuberances, is 

 hairy. The bicolor pattern of coloration is exactly as in Vesperimus. 



This section is based upon the Hypuditus leucogaster of Maximilian, after- 

 ward the Mus missouriensis of Audubon and Bachman. To the original type- 



