158 
JOURNAL OF MAMMALOGY 
specimen from Las Cruces, however, it is absent on the left side only. 
Here the anterior premolar {jpmP) as if taking advantage of the loss of 
the middle one, is noticeably larger than the anterior premolar of the 
opposite side and stands fully in the tooth row so as to fill the space 
completely between canine and last premolar On the opposite 
(left) side, the anterior premolar lies just inside the axis of the tooth 
row, while the middle premolar (pm^), much smaller, is forced even 
more inward. 
This tendency to lose certain of the teeth is seen in many and very 
remotely related species of bats. It is not necessarily a result of the 
gradual shortening of the tooth row to produce greater crushing power 
for as in the Las Cruces specimen, the anterior premolar of the right 
side filled the space of two on the opposite side. The frequency with 
which the middle premolar is missing in this species indicates that the 
upper dentition is now in course of evolution from a three-premolar 
to a two-premolar stage. Apparently, to judge from the available 
specimens, the latter, more reduced condition has made such headway 
as to be now as frequent as the three-premolar stage. Should it even- 
tually prevail within the species, the normal dentition would then have 
the same formula as in Lasionycteris, Corynorhinus, and certain other 
genera, so that it might come to be placed in a genus distinct from 
Myotis. Thus we may have in Myotis occultus a genus in the making, 
a species now referred unhesitatingly to Myotis, but which, if the loss 
of the middle premolar became permanent, would perhaps come to be 
regarded as a monotypic genus, characterized by its reduced dental 
formula. 
Pipistrellus hesperus (H. Allen) 
WESTERN BAT 
This little whitish species with its black contrasting membranes, is 
confined more or less closely to the Great Basin. Although probably 
common locally, there is but one specimen in the collections here re- 
ported on, a male taken October 1, on the west side of the Organ Moun- 
tains, New Mexico, by W. Huber. 
Eptesicus fuscus (Beauvois) 
LARGE BROW’N EAT 
At Las Cruces, New Mexico, Mr. Huber found a considerable colony 
of this common and widespread species living in the roof of a building 
