152 MONOGRAPHS OF NOKTII AMERICAN ltODENTIA. 



;is long as head to nearly half as Long as trunk. Pelage thick, soft, of ordinary 

 fur, interspersed dorsally with longer hristly hairs; the Curat the roots uniformly 

 plumbeous, colored only at the tips; general colors subdued, the shades 

 diffuse, 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 lour groups, as originally 

 pointed out by Professor Baird. These groups may be thus conveniently 

 analyzed : — 



(a) Back upper molar with 2 external triangles and a posterior 



crescent Myonomes. 



1 external triangle and a posterior 



trefoil (b). 

 (b) Front lower molar with 3 internal and 2 or 3 external closed 



triangles Cliilotus. 



2 internal and 1 external closed tri- 



angle (c). 



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



obviously larger than hind claws Pitymys. 



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 distinguish 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, combining dental and external characters, we may draw up the 

 following antithetical diagnoses, making combinations perfectly tangible and 

 satisfactory : — 



A. — Myonomes. Back upper molar with 2 external triangles and a posterior 

 crescent. Middle upper molar with 2 internal triangles. Front lower 

 molar with 3 internal and 2 or 3 external lateral triangles. Ear unrim- 

 med in front. Sole 6-tuberculate. Fore claws not longer than hinder 

 ones. Tail about : ' { the head and body, or more. Pelage ordinary. Of 

 maximum and medium size. 



