THE FAMILIES AND GENERA OF BATS. 
21 
^ In Nyctinomus brasiliensis , with corresponding 
part of permanent formula the same as in Lasiurus , it also appears to 
be present, though I have been unable to verify the occurrence of mi z . 
- 2 3. 1. 1 2 
A slightly reduced formula _* ^ a PP ears to be charac- 
teristic of the Desmodontidse and Phyllostomidse, though the pos- 
terior tooth both above and below is occasionally so small that its 
presence can not be positively determined. In Desmodus the doubtful 
tooth is mp 2 , while in Pliyllostomus it is mp 2 . In two specimens of 
Artibeus (Nos. 38309 and 38310, U. S. Nat. Mus., Old Providence 
Island, Carribbean Sea) , on the other hand, I can find no trace of 
mp x . F or the Megachiroptera the complete milk dentition is prob- 
ably the same as that of the Phyllostomidse. At least, I have verified 
— 2 3. 1. 1 2 
the formula j— ^ — * 1* 1 * n ^V no I > ^ eTUS anc ^ Pterocyon. In both 
of these the posterior tooth above and below corresponds with pm 4 , 
and pm 4 , respectively, pm 3 and pm 3 being conspicuously without a 
preceding functional deciduous tooth. 
In form the milk teeth are for the most part quite unlike those of 
the permanent dentition. They are usually 
slender, minute spicules, with straight or re- 
curved tips, and their only function is prob- 
ably, as has been frequently suggested, to aid 
the young in clinging to the mother during 
flight. In size they are so insignificant that 
one may not infrequently persist in the edge of 
an alveolus until the permanent tooth is nearly 
grown. Ihese peculiarities ot size and form, cynopterus minutus, Nias 
together with the very early development of island, no. 141271. x 
, .. . . ^ . ABOUT B. 
the permanent dentition, preclude all possi- 
bility that the milk teeth can be used in mastication, and indicate 
that, whatever may be the case with other mammals, in bats the de- 
cidous teeth can have had little if any influence on the modifications 
of the permanent set. 
The simplest form of milk tooth is a straight terete spicule, tapering 
to a sharp point, and with no evident differentiation between root and 
crown. This is found in the lower cheek teeth of Cynopterus. The 
next and more usual type differs in the definite recurving of the point. 
This curve may be slight and gradual (canines and cheek teeth of 
Cynopterus (fig. 1) and Pterocyon , upper canine of Myotis yuman- 
ensis) or strong and abrupt, sometimes almost angular (canines and 
cheek teeth of Phyllostomidse, canines and incisors of Nyctinomus 
brasiliensis) . Occasionally this type is further modified by a slight 
but evident shortening and thickening of the crown, or the faint indi- 
