102 PROCEEDINGS OF THE NATIONAL MUSEUM vol. no 



On the basis of an abnormal supernumerary deciduous incisor in 

 Myotis lucifugus, Stegeman (1956, p. 60) postulated that the normally 

 missing tooth may be I^. G. Allen (1916, p. 335) described a specimen 

 of Plecotus townsendii (USMN 150273) with three upper permanent 

 incisors. The "extra" incisor was the outermost of the three and 

 differed in form from the other two. Allen speculated that this 

 might represent a long lost P. 



The uniform absence of a tooth throughout a group as large and 

 as varied as the Chiroptera compels the impression that the absence 

 is a characteristic dating from the dawn of development of the group 

 and subsequently shared by all derivatives. It is doubtful that 

 deviations from such a deep-seated characteristic, such as the devia- 

 tions reported by Allen and by Stegeman, can be assumed to represent 

 a reversion to an even more primitive long lost characteristic. 



An example of a clearly abnormal extra incisor may be seen in 

 a specimen of P. mexicanus (KU 29858) . In place of the right outer 

 upper incisor there are two closely approximated twin teeth, neither 

 of which resembles the normal outer incisor in the left premaxilla. 



Investigation of the factors now operating to cause reduction and 

 loss of the remaining incisors in Recent bats may give clues to the 

 incisor missing throughout the order. As noted above. Miller, 

 following Winge, supposed that reduction of the inner portion of the 

 premaxillary bone left the innermost incisor without a place for its 

 root. However, although all bats lack one upper incisor, not all bats 

 have the tooth-bearing portions of the premaxillaries reduced. There- 

 fore, if the same incisor is missing in all bats, then reduction of the 

 premaxillaries can not always account for the loss. 



Actually, several evolutionary trends may account for crowding 

 and subsequent reduction and loss of incisors. In forms with an 

 extensible tongue, the action of the tongue may be a responsible agent. 

 In others, a narroAving of the rostrum and mandibles, an enlargement 

 of the canines, or a reduction of the tooth-bearing portions of the 

 premaxillaries may be a cause. 



The result of these processes is always the same. It is not the 

 reduction and loss of the innermost of the two remaining incisors, but 

 a crowding of both toward the canine. A consequence of this crowding 

 is conflict of the outermost incisor with the occluding lower canine. 

 Further alternative consequences are reduction of the lower canine, 

 diminution or obliteration of the outermost upper incisor, anterior 

 or posterior displacement of the outermost incisor, rotation of the 

 outermost incisor to a narrower axis, elevation of the rostrum, or 

 depression of the mandible. All of these conditions may be observed 

 in Recent bats. It is important to note that in all Chiroptera it is 

 invariably the outer of the two remaining incisors that is reduced in 



