246 DURHAM 



Powell Spring. Apparently they occur in certain habitats on the North 

 Rim where the deer mouse is rare or absent. 



Size: These adults are considerably larger than the topotypes from 

 Utah (Osgood, 1909, p. 145) which are similar in size to others from 

 Utah (Durrant, 1952, p. 320), and those from Colorado and New Mex- 

 ico (Warren, 1942, p. 205, and Bailey, 1931, p. 154, respectively). 

 These from the North Rim are even larger than those large specimens 

 from southeast Nevada which were thought by Hall (1946, p. 519) 

 to be merely individual and geographic variants. The North Rim form 

 is slightly larger than the Hancock series of four adult males and three 

 adult females from the South Rim. More specimens are needed from 

 the North Rim, but apparently the population there is unique in having 

 (1) large size (head-body length of adults, 101 mm or longer) and (2) 

 long rostrum as expressed in (A) actual measurements of greatest length 

 of skull, (B) length of nasals, and (C) relative lengths, i.e., ratio of 

 nasals to greatest length of skull and ratio of nasals to basilar length. 

 In the males of the North Rim the nasals are 41.3 per cent of greatest 

 length of skull whereas the averages for the larger sex from the South 

 Rim, from Nevada (Hall, op. cit.), from Southern California and from 

 Utah (Durrant, op. cit.) are 40, 39, 39, and 37.3 per cent, respectively. 

 In the males of the North Rim the nasals are 56.6 per cent of the basilar 

 length, whereas in the above populations the percentage is 53.8, 51.6, 

 51.9 and 49.7, respectively. In length of head-body and size of skull 

 and in relative lengths of nasals to greatest and basilar lengths of skull, 

 the population of the North Rim agrees with those of the two large mice 

 P. b. attwateri of Texas and P. h. artemesiae of Wyoming. 



Comparisons: Because the brush mouse P. b. rowleyi of the North 

 Rim has an unusually long nose, the specimens were carefully examined 

 to be sure that they were not P. nasutus, the long-nosed deer mouse. The 

 actual and relative length of nasals of the males of the brush mouse from 

 the North Rim exceed the maximum measurements for the long-nosed 

 deer mouse in Colorado and New Mexico. To my knowledge, P. nasutus 

 does not occur westward and northward of the Colorado River. 



The P. b. rowleyi of the North Rim differ from the Hancock series 

 taken in Los Angeles vicinity in having darker dorsal pelage, less buff 

 on sides, longer rostrum and nasals, and shorter premaxillaries. Pre- 

 maxillaries in the population from the North Rim extend posteriorly 

 to the zygomatic branch of the maxillary but not as far as the proximal 

 end of the nasals. Topotypes of P. nasutus griseus from New Mexico 

 (in the Los Angeles County Museum) have premaxillaries which extend 



