MAMMALS ()K T1IK MKXK AN BOUNDARY. 363 



K v.x I'd TEN GEN BR \ OF Ml RID V." 



a. < Grinding teeth crowned by tubercles or cusps. 



I>. Grinding teeth with tubercles arranged in three transverse rows, verydistincl in teeth 



of upper jaw M us (p. 363). 



hi). Grinding teeth with tubercles arranged in two longitudinal rows. 



c. Upper incisors longitudinally grooved -Ri ithrodqjitamys \>. 156 ). 



cc. Upper incisors no! grooved. 

 ./. Skull with a distinct ridge over eye-socket; fur coarse; belly nol pure white; size 



larger (total length usually over L30 mm) Oryzomys (p. 154 . 



(/</. Skull without a prominent ridge over eye-socket: belly pure white: smaller 

 (total length usually under 130 nun). 

 e. Coronoid process of mandible developed as a long hook: sole< of feet hairy. 

 /. Body stout: size medium to rather large dotal length 125 to 17."> mm.): 

 plantar tubercles, four: teeth with high tubercles. . . .Onychomys i p. 368). 

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

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



<i. Coronoid process of mandible developed as a short spine: soles of feet largely 



naked .Peromyscus ' p. 382 1. 



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

 of enamel inclosing dentine. 



g. Enamel loops S -shaped Sigmod m I p. 145). 



g<j. Enamel loops distinctly triangular. 



h. 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 I p. 467 i. 



lih. 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). 



ii. Tail round Wicrotus I p. 198 



Subfamily MTTRHST^TC. 



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 (1758). 



IS 



Dentition— I. \ \ ; M. \ "K». U^ 



n be 



Front teeth two: check teeth never r,,, > -.; , - \.lexandrinub. 

 more than three in each jaw. Incisors " s K < " ; ''■ UppER MOLARS : 



J C, LOWEB MOLARS. 



narrow, without grooves. Structure <>l 



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

 ess of mandible well developed. Ears and eves 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 Slate Museum, VIII, October, 1900, p. 93. 



