Missing Premolar of the Chiroptera. 347 



Tlitlitrto authors have taken for granted tliat tlic anterior 

 tooth, the protus * or jj^, was the missing one ; but they 

 appear to have done this ratlier because it was the simplest 

 theory than tliat tliey had any strong reason for it. Even 

 "Wiiige, \\\\o gives in most cases such full reasons for his 

 cnnchisioiis, merely says " it is usually presumed that it is 

 l?- which is absent " f. 



In such cases the arguments that are available are of three 

 kinds, viz. : (1) relative position in the jaw, (2) ihe occasional 

 aecurrence of atavistic teeth, and (3) ttie presence or absence 

 of milk-teeth corresponding to the permanent ones. The first 

 two of these arguments may often be fallacious, while ihj 

 third is a very important one ; but if, as now, all three agree 

 in pointing to one conclusion, that siiould be accepted even if 

 it differs from the usual opinion on the subject. 



It is, of course, certain that the two posterior premolars of 

 bats are to be homologized as jy" and p^, tritus and tetartus; 

 and the question to be settled is as to whether the most 

 anterior one is the protus or deuterus, p^ or p"^, and I have 

 come to the conclusion that this tooth is the protus and that 

 the deuterus is missing, for the following reasons : — 



(1) As to relative position, attention may be drawn to ths 

 way in which the anterior tooth in Pterocyon helvus, \n 

 Lonchoglossa, and others, stands close behind the canine, 

 with a gap separating it from the other teeth. 



(2) Dr. K. Andersen has shown me a skull of Pieropus 

 scapulatus (B.M. no. 86. 11. 1. 1) in which the mandible 

 possesses on one side a well-developed tooth standing in the 

 gap thus formed, and, 1 would suggest, representing the 

 missing j(y2. The additional premolar described by Peters;}: 

 in a specimen of Anvura geoffroiji is again in an exactly 

 similar position, and may be equally of an atavistic nature. 



(3) The really important test as to whether a tooth is a. p^ 

 ox p^ is, among the l^V-rte, as to whether it does or does not 

 have a milk predecessor, no protus in the group being knowu 

 to change §, while the deuterus is always represented by bjtli 



* Cf. P. Biol. Soc. Wash, xviii. p. 196 (1905). 



t " Pattedyrenes Tandskifte," iu ^'id. Medd. Nat. For. Copenhagen, 

 1882, p. 02. 



X MB. Ak. Berl. 1869, p. 398. 

 § A case contradicling this rule 



4 iliX^. Xin. ^CJX. ,.^KJKJ, (.. XJ^^. 



§ A case coiitradiciing this rule would appear to be represented by the 

 mole's dentition a.s described by Tauber (Naturh. Tidsskr. (3) viii. p. '2'y2, 

 pi. xi., 1872), but, judjrinp: by the tigure, his interpretations are palpably 

 iucoiTect. Taking his own diagram, no one could hesitite in deciding 

 that the teeth he calls mil. 1 in the upper jaw and ;wj. 1 in the lower 

 correspond absolutely with each other, instead of one being milk and the 

 other peruuiueut. The true explanation of his drawing is evidently that 



