20 Osgood — Four Neio Pocket Mice. 



mens from Needles, California, are typical of haiigsi and in the compara- 

 tively short distance between that locality and Yuma no specimens of this 

 group have been taken. Possibly the Colorado River effectively separates 

 the ranges of the two species. P. hombycnius is represented by tlie type 

 and two Sonoran specimens, one from Colonia Lerdo, and one from the 

 Sonora Mesa near the Colorado River twenty miles south of the interna- 

 tional l)oundary. An immature specimen from Trinidad Valley, north- 

 west base San Pedro Martir Mountains, Lower California, may be pro- 

 visionally referred to this species. Its color is rather dark but its badly 

 shattered skull shows large mastoids. 



Perognathus penicillatus siccus subsp. no v. 



T'ype from Ceralbo Island, Lower California, Mexico. No. 146,890 U. S. 

 National Museum, Biological Survey Collection, d* adult. February 13, 

 1906. E. W. Nelson and E. A. Goldman. 



Characters. — Size decidedly larger than in P. }>. (irenuriuK ; very weak 

 rump bristles rarely present; color dimorphic, buff phase slightly darker 

 than in arenarius, gray phase decidedly different ; skull large and heavy ; 

 mastoids rather large. 



Color. — Buff phase: Practically as in arenurius but averaging slightly 

 darker; general effect of upperjjartsbuffy fawn; lateral line narrow, pink- 

 ish buff; underparts creamy. Gray phase : Similar to buff phase, but gen- 

 eral effect of upperparts ranging from drab gray to broccoli brown. 



Skull. — Similar to that of arenarius but decidedly larger and heavier; 

 mastoids ratlier large ; ascending branches of supraoccipital broad; similar 

 to that of aminophilus but averaging larger with relatively large mastoids. 



Measurements. — Average of ten adult topotypes : Total length, 175 (165- 

 187) ; tail vertebrje, 98 (92-102) ; hind foot, 24.5 (23.5-26). Skulls of type 

 and one topotype, respectively: Greatest length, 25.9 ; 26.9; basilar length, 

 17.7; 18.9; mastoid width, 13.8; 13.8; zygomatic width, 12.6; 13; inter- 

 orbital constriction, 6.6 ; 6.6; nasals, 8.9; 9.6; interparietal, 7.2 x 3.6 ; 7.5 

 x3.8; diastema, 6; 6.7; maxillary toothrow, 4 ; 3.9. 



Remarks. — This form was found not only on Ceralbo Island but also at sev- 

 eral localities on the neighboring end of the Peninsula. Specimens from 

 the peninsular localities Tres Pachitas and Pescadero seem referable to it, 

 while others from slightly farther north show intergradation with arena- 

 rius. A series from San Jorge, the type locality of arenarius, consists largely 

 of intermediates between .siccus and a smaller form of the central part of the 

 Peninsula. The type of arenarius, however, is decidedly referable to the 

 smaller form and the majority of the topotyi)es are nearer to it than to 

 siccus. 



Careful examination reveals a few very weak rump bristles in several 

 specimens of s/ccn.?, though they are not found in other members of the 

 penicillatus series. 



Perognathus penicillatus ammophilus subsp. nov. 



Type from Margarita Island, Lower California, Mexico. No. 146,859 U. S. 

 National Museum, Biological Survey Collection, c? adult. November 29, 

 1905. E. W. Nelson and E. A. Goldman. 



