MAMMALB OF THE MEXICAN BOUNDARY. 



363 



KEY TO TEN GENERA OF MUKID^.O 



a. Grinding teeth crowned by tubercles or cusps. 



h. Grinding teeth with tubercles arranged in three transverse rows, very distinct in teeth 



of upper jaw Mus (p. 363). 



bb. Grinding teeth with tubercles arranged in two longitudinal rows. 



c. Upper incisors_ longitudinally grooved Reithrodontomys (p. 45fi). 



cc. Upper incisors not grooved. 



d. Skull with a distinct ridge over eye-socket; fur coarse; belly not pure white; size 



larger (total length usually over 1.30 mm) Oryzomys (p. 454). 



dd. Skull without a prominent ridge over eye-socket; belly pure white; smaller 

 (total length usually under 130 mm). 

 e. Coronoid process of mandible developed as a long hook; soles of feet haiiy. 

 /. Body stout; size medium to rather large (total length 125 to 175 mm.); 



plantar tubercles, four; teeth with high tubercles Onychomys {p. 368). 



ff. Body mouse-like; .size small (total length less than 100 mm.); plantar tuber- 

 cles, six; teeth with low tubercles Baiomys (p. 381). 



ee. Coronoid process of mandible developed as a short spine; soles of feet largely 



naked Peromyscus (p. 382). 



aa. Crowns of grinding teeth flattened, and divided into loops or triangles formed b}' plates 

 of enamel inclosing dentine. 



g. Enamel loops S-shaped Sigmodon (p. 445). 



gg. Enamel loops distinctly triangular. 



Ji. Upper incisors narrow, compressed, the antero-posterior diameter of each 

 much greater than the transverse diameter; body slender; tail always 



long; eyes and ears large; belly white Neotoma (p. 467). 



hh. Upper incisors broad, the antero-posterior diameter of each less than 

 transverse diameter; body clumsy; tail usually short; eyes and ears 

 small; belly generally not white. 



i. Tail flattened laterally Fiber (p. 494). 



a. Tail round Slicrotus (p. 498) 



SubfaiTiily :M:TJRIT»rJK!. 



Molars rooted and tuberculated, those 

 of the upper jaw with three longitudinal 

 rows of tubercles. The species existing 

 in the New World have been introduced 

 and naturalized from the Old World.- 



Genus MUS Linnaeus (17B8). 

 Dentition— I. ^ ; M. |rr| = 16. 



!■■ - a be 



Front teeth two; cheek teeth never fig, ss.-mus alexandrinus. 

 more than three in each jaw. Incisors ''• skull; b, upper molars; 



'' j> '^' Lower molars. 



narrow, without grooves. Structure oi 



molars as in fig. 58. Incisive foramina of skull long; coronoid proc- 

 ess of mandible well developed. Ears and eyes rather large. Muzzle 

 naked at the extremity. Fur soft, in some cases mixed with spines. 



a Largely borrowed from Miller's Key to the Land Mammals of Northeastern North 

 America, in Bulletin of the New York State Museum, VIII, October, 1900, p. 93. 



