206 DR. KNUD ANDERSEN ON BATS. [Apr. 7, 



nothing or, if preferred, anything ; attempts to determine the 

 homologies of teeth in Mammalia on the basis of the corre- 

 spondence of the upper with the lower teeth, or vice versa, would 

 in too many cases lead to obviously absurd results. As to the 

 latter argument, it seems to me, on closer examination, to lead to 

 precisely the opposite conclusion. The tendency for the pre- 

 maxillaries to become redviced along their inner edge is fii-st 

 developed in the higher of the two suborders of Bats, the Micro- 

 chiroptera ; in the Megachiroptera no such tendency obtains, and 

 nevertheless they have only two pairs of upper incisors. When, 

 therefore, one pair has been lost also in those primitive bats in 

 which the premaxillaries have not been reduced along their inner 

 edge, the loss of this pair must evidently be due to some other reason. 

 The strong lower canines in bats (or rather their ancestors) have 

 probably effected the degeneration, and ultimate disappearance, 

 of that pair of upper incisors which, if it were present (or present 

 in its full size), would hinder their free passage in front of the 

 upper canines ; in other words, in passing in front of the upper 

 canines the lower canines have checked the growth, and 

 ultimately caused the complete disappearance, of i^. In accord- 

 ance with this we find in most Megachiroptera the four upper 

 incisors (i^ i^ i^ i") close together, but i" separated by a wide 

 diastema from the canine ; part of this diastema indicates the 

 former place of i', but it has no doubt been widened to allow of 

 the free action of the lower against the upper canines. In many 

 genera of Microchiroptera, this line of development has been 

 carried a step fiirther ; by a narrowing of the diastema between 

 i^ and c the former has come closer to the latter and within reach 

 of the lower canines, which then cause a decrease in size (and 

 change in shape) of i". This is the case in the three genera 

 which form the subject of the present paper, and which, there- 

 fore, in showing what actually takes place, in living bats, with 

 regard to i~, gi^^^e, so to say, an illustration of what has probably 

 taken place, in the ancestors of bats, with regard to the now 

 permanently lost i^. 



A large number of Chiropteran genera (some 60 out of the now 

 recognised 173 genera) have three pairs of lower incisors; in 

 most of these genera the lower incisors are subequal in size ; 

 in those few in which one pair is noticeably, or even considerably, 

 reduced, this pair is i., (compare f . i. Bhogeessa, Baiodon, some 

 species of Nyctinomus, MormojJtet^its). From this it appears safe 

 to assume that in bats which have only two pairs of lower incisors, 

 the missing pair is i^. 



No bat has more than three premolars, above and below. As 

 recently pointed out by Oldfield Thomas* the permanently missing 

 Lipper and lower premolar is in all probability p' and p, (not, as 

 hitherto taken for granted, p^ and pj. 



In those Phyllostomatidse which have three upper premolars 



* Oldfield Thomas, " The Missing Premolar of Chiroptera," Ann. & Mag. N. H. 

 (8) i. pp. 346-348 (April, 1908). 



