1918] Grinnell: A Synopsis of the Bats of California 243 
than that of non-volant mammals; but were each of these islands to 
exhibit a unique condition of temperature and humidity, it may be 
ventured that the bats inhabiting them would be found to represent 
distinct races, Just as do birds in similar eases. 
In California there is not a single obvious barrier to the distri- 
bution of any species of bat; yet not one of the thirty-one forms 
inhabiting the state has been found to be distributed uniformly 
throughout the entire area. In the most abundantly represented 
genus, Myotis, at least fourteen forms are to be recognized, each 
strictly limited in the breeding season to a certain zonal and faunal 
area. To illustrate the point: Myotis orinomus has never been taken 
outside of the semi-arid region of the higher portion of the Upper 
Sonoran zone. 
DENTITION 
Young bats are born with a complete milk-dentition, the function 
of which is probably to enable them to hold on to their mothers. These 
teeth differ both in number and form from those cf the permanent set. 
They are homodont, slender, and sharply recurved, and resemble the 
teeth of seals and cetaceans more nearly than those of adult bats 
(Barrett-Hamilton, 1910, p. 15). 
The permanent teeth consist, as in other mammals, of four kinds: 
incisors, canines, premolars, and molars. These teeth differ widely 
among the different genera and species, in both numbers and form, 
there being, for example, but twenty teeth in the jaws of the blood- 
sucking genus Desmodus, and these so specialized that it would be 
impossible for the animal to masticate; while the highest number of 
teeth present in any known bat is thirty-eight (Miller, 1907, p. 23), 
the number present in the genus Myotis. 
In several of the species here listed some of the teeth are so minute 
as to be functionless and are difficult to find; in fact they sometimes 
drop out of the jaw. In Nyctinomus mexicanus, for instance, the 
number of incisors appears to be variable; and in Myotis occultus 
the middle upper premolar, normally present among related species, 
is lacking in half the known specimens. 
CoLORATION 
In common with other nocturnal or crepuscular mammals bats 
have, as a rule, a much more sober coloring than diurnal mammals, 
