1914] GrinneU: Mammals and Birds of the Colorado Valley 265 



I sent oue example to the Bureau of Biological Survey, where Mr. 

 A. H. Howell made the specific determination here employed. Our 

 series bears out closely the characters assigned in the original descrip- 

 tion (Hollister, 1909, p. 43), and they are in all these respects sur- 

 prisingly uniform. The very broad and flat-topped ro.strum and 

 braincase constitutes a character for discrimination from all other 

 species of Myotis in California except orinomus. The proportions 

 generally are peculiar (see table of measurements). 



Fig. H. Right upper dental series Fig. I. Eight upper dental series 



of Myotis occultus, no. 10702, 2; mid- of Myotis occultus, no. 10706, (J; mid- 



dle upper premolar absent. X 4. die upper premolar present. X 4. 



Variation in general proportions from 

 those of preceding figure possibly due 

 to age. 



An interesting fact pointed out by Hollister is the variability in 

 a feature usually considered of much more fundamental importance 

 than the external characters employed in distinguishing members of 

 the genus, namely, the presence or absence of the middle upper pre- 

 molar (pm'). In one of Hollister 's specimens this tooth was present, 

 in the other absent ; in three of ours it is present, in three it is wanting. 

 Thus fifty per cent of the individuals so far collected lack the tooth 

 in question, certainly a remarkable aberrancy from the norm in the 

 genus Myotis (see Miller, 1907, p. 201), and denoting a tendency to 

 specialization in this member of the genus, along a line regularly 

 shown in other closely related genera (see figs. H, I). 



Myotis californicus pallidas Stephens 

 Stephen.s Little Pallid Bat 



The accompanying table shows certain circumstances of capture of 

 this species. Although obtained at but the two localities, Mellen, on 

 the Arizona side, and opposite The Needles, on tlie California side. 



