54 Osgood — Haploviylomys, a New Subgenus of Peromyscus. 



the restricted genus Peromyscus, typified by the common P. 

 leuco'pus of the eastern United States. 



A small group containing two well-known species and numer- 

 ous subspecies found in the arid and semi-arid regions of the 

 southwestern United States and northern Mexico seems also 

 worthy of subgeneric recognition. Although not differing as a 

 group in any external characters that are diagnostic, it is sharply 

 defined by peculiarities of the molar teeth, which are so con- 

 stant and, comparatively speaking, so pronounced as to be of 

 consideraV)le significance. 



The important forms of this group are P. eremicus and P. 

 californicus, characterized by a less complex tuberculation of the 

 molar teeth than in Peromyscus proper or in Megadontomys. In 

 the ordinary type of Peromyscus there is a small accessory tubercle 

 between the primary outer tubercles of the first and second upper 

 molars. In unworn teeth these tiny tubercles are scarcely notice- 

 able, except as viewed in profile, ^^'hen the crowns of the molars 

 become worn, however, they appear as narrow enamel loops with 

 closely appressed sides, lying between the more or less open 

 ])rimary loops. These small tubercles are not ]>resent in the 

 group heretofore loosely called the 'eremicus' group. Tiiey 

 are also absent in Baiomys, which, however, is otherwise i)eculiar. 

 They are developed to various degrees in various species, in some 

 being difficult to observe, except in teeth that have been sub- 

 jected to considerable wear. Ap])arently they are least promi- 

 nent in P. crinitus and its close allies. 



Tlie appearance of partly worn teeth is shown in tlie accom- 

 panying reproduction of photographs of actual specimens. For 

 l)urposes of illustration, two of the larger species were selected. 

 In essential characters their teeth do not differ from those of the 

 type species of their respective groups. 



The new subgenus may be characterized as follows : 



Haplomylomys subgen. nov. 



Type. — Peromyscus eremicus (Baird), from Fort Yuma, California. 



Characters. — Size medium or small ; pelage usually very soft and silky ; 

 tail longer than head and body, subterete, rather thinly haired; soles of 

 hind feet naked (at least in median line) to calcaneum, 6-tuberculate and 

 paved with minute imbricate scales ; skull with cranium rather large and 

 rostral region relatively weak ; first and second upper molars with three 

 salient and two reentrant outer angles at all stages of wear; small 

 secondary tubercles never present between outer primary tubercles ; lower 

 molars correspondingly simple. 



