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152 JlONOOUAl'IIS OF NORTH AMERICAN ItODENTIA. 



as loiif,' lis lu'iul fo iirarly Imlf as loii",' as trunk. Pelage thick, soft, of ordiiuiry 

 fur, iuhirspcrsf-'d dorsally witli longer bristly hairs; the fur at the roots uniformly 

 l)lunil)oous, colored only at the tips; general colors subdued, the shades 

 diiluse, rarely with distinct parti-coloration, but under parts ordinarily lighter 

 than upper, and tail bicolor from sharing the respective shades. 



The North American species fall naturally into four groups, as originally 

 ])ointed out by Professor Baird. These groups may be thus conveniently 

 analyzed : — 



((f) ]}ack u})per molar with 2 external triangles and a posterior 



crescent Mijonomes. 



1 external triangle and a posterior 



trefoil {h). 

 {!>) Front lower molar with 3 internal and 2 or 3 external closed 



triangles Chilolus. 



2 internal and 1 external closed tri- 



angle (c). 



(c) Fore claws not larger than hind claws Pedomys. 



obviously larger than hind claws . Fitymys. 



There are no tangible differences in the skull and teeth between Pedomys 

 and Pitymys ; nevertheless, the external peculiarities of Pitymys are greater 

 than those of either of the other forms. Chilotus, in general external char- 

 acters, is so like both Myonomes and Pedomys that we cannot diatinguish them, 

 except by the peculiar construction of the ear. It shares the posterior trefoil 

 of the back upper molar with Pedomys and Pitymys, but shares the greater 

 number of lateral closed triangles of the front lower molar of Myonomes. 

 Myonomes, again, is not separated from Pedomys by any external character, but 

 by its posterior crescent and two external lateral triangles of the back upper 

 molar. So, coml)ining dental and external characters, wc may draw up the 

 following antithetical diagnoses, making combinations perfectly tangible and 

 satislactory : — 



A. — MvoNOMiis. Back upper moiar with 2 external triangles and a posterior 

 crenent. Middle upi)er molar with 2 internal triangles. Front lowet 

 molar wi' I) 3 internal and 2 or 3 external lateral triangles. Ear unrim- 

 med in front. Sole G-tuberculate. Fore claws not longCi th;.i hinder 

 ones. Tail about J the head and body, or more, Peliige ordinary. Of 

 maximum and medium size. 



