CHIROPTERA 15 



both in number and form from those of the permanent dentition. 

 They are homodont, slender, sharply recurved and cusped, and 

 cannot by their shape be divided into the ordinary divisions 

 found in the adult — incisors, canines, premolars, and molars — 

 but rather resemble the teeth of seals and cetaceans. They are 

 weak and insignificant, and may persist in the edges of the 

 alveoli until the permanent teeth are nearly grown ; ^ their 

 number for each species is not known, but in Myotis it is — 



.2-2 i-i . 2-2 



mi , mc , mpm =22, 



3-3 I- I ^ 2-2 



a combination which probably represents the maximum in cor- 

 respondence with the high number of the teeth of the per- 

 manent dentition in this genus. 



The permanent teeth consist, as in other mammals, of four 

 kinds — incisors, canines, premolars, and molars. The cheek- 

 teeth are acutely cusped, with a pattern more or less W-shaped. 

 They vary in the different genera from thirty-eight in Myotis to 

 thirty-two, as in Vespertilio and Rhinolophus. In most genera 

 one or more of the teeth are so minute as to be functionless, 

 difficult to find, and indeed they sometimes drop out of the jaw 

 of the adult animal, so that the total, as given in the technical 

 formulae, is often deceptive to the student. The molars, the 

 last three teeth in either jaw, are not affected by this reduction, 

 neither are the canines at the other end of the cheek-series, 

 nor, except in the Rhino lop hi dcE, the incisors. The teeth, 

 therefore, which most call for careful study are the premolars, 

 and they, fortunately, are easily recognised, since they are all 

 the cheek-teeth between the last three (the molars), and the 

 first, the conspicuous canine. 



If we accept the current view of evolutionists, that the 

 tendency of mammalian dentition lies towards reduction in the 

 numbers of individual teeth, then the genus Myotis possesses 

 the lowest type of dentition, the full formula in this case being — 



pm ^ — ^, m ^ — ^ = 38. 



3-3 i-i 3-3 3-3 



1 For further details see L. F. E. Rousseau, "Mem. Zool. et anatomique sur la 

 Chauve-Souris commune," etc., in Guerin's Mag. de Zool., 1839, pi. 7 ; also Wilhelm 

 Leche in Lund's U?nversitets Ars-skrift, xii., 1-47, 1876, pis. i and 2 (analysed in 

 Archiv filr Naturgeschichie, i., 1877, 353-364), and xiv., 1-37, 1878, pis. i and 2. 



