RHITHRODONTOMYS. 257 



groove, nearly as broad as the face of the tooth; lower incisors simple; 

 anterior upper molar with four roots, three large, one very small. 

 The lower half of the descending ramus is abruptly twisted inward 

 nearly at a right angle to the lower border of the process; antor- 

 bital foramen is situated in the zygomatic portion of the maxillary 

 and is almost circular above, contracting to a slit below; palate 

 terminates opposite the posterior border of last molars as a trans- 

 verse shelf; bullae large, widely separated, but approximating from 

 behind anteriorly. 



KEY TO THE SPECIES AND SUBSPECIES. 



A. Tail about one-half the total length. PAGE 



a. Above yellowish brown lined with black; 



beneath ashy R. aitstralis 259 



b. Above sepia, shading to Isabella; beneath 



Isabella R. a. rulcanius 259 



c. Above mouse gray; beneath yellowish or 



grayish white R. megalotis 259 



d. Above gray, strongly varied with black; be- 

 neath white R. m. scstincnsis 260 



e. Above dull fulvous brown; beneath buffy 



salmon R. m. zacateccs 260 



f. Above black and rufous; beneath buffy 



salmon R. m. obscurus 261 



g. Above yellowish brown and black; beneath 



grayish white R. longicaiidus 261 



h. Above buffy gray; beneath white R. I. pallidns 262 



i. Above dark brown and black ; beneath whitish 



plumbeous R. saturates 262 



j . Above yellowish brown and black ; beneath 



whitish plumbeous R. s. cinereus 262 



k. Above fulvous brown lined with black; be- 

 neath silver gray R. s. alticola 263 



B. Tail more than half the total length. 



a. Above reddish brown R. peninsula; 263 



b. Above fulvous and black; beneath pale ful- 

 vous, including chin and throat R. difficilis 263 



c. Above rufous brown; beneath white R. incxicunns 264 



d. Above grayish brown and yellowish ; beneath 



white R. m. intcrmcdius 264 



e. Above yellowish brown and black; beneath 

 whitish. 



a/ Large; total length, 183 mm R. m. fulvescens 264 



