1896.] MAMMALIAN DENTITION. 591 



compose the order Insectivora, we find a considerable variation in 

 their dentition, both as regards the miinbor of their teeth and the 

 specialization of the individual members of the dental series. 



Thus in Gymnura and Talpa we find in the adult the full pla- 

 cental dentition of 44 teeth, while in two Shrews (Diplomesodon 

 and Anurosorex) the dentition is reduced to 26 teeth, other families 

 presenting numerous stages intermediate between these two. If 

 primitive, the supposed presence of 4 upper iucisiors in Sorex and 

 the 4 upper molars of Cenletes must be of great interest, but the 

 former I believe is capable of being interpreted differently, and the 

 latter to be a secondary character. 



A closer examination of these dental variations shows that they 

 oan be grouped under four heads : — 



(1) A tendency for a suppression of the 3rd incisor above and 



below, di. 3 disappearing first. 



(2) A suppression in the premolar series, pm. 1 in the Cen- 



tetidce, pm. 2 in Selenodon. 



(3) A suppression of the posterior molars, the number vary- 



ing from J to f, the normal number being |-. 



(4) A tendency for reduction in the functional importance of 



the milk dentition. 



Although representatives of only 5 out of the 9 families of the 

 Insectivora (Mower and Lydekker, 4) have been systematically 

 examined, this last variation is so marked, that one is forced to the 

 conclusion that the order as a whole is tending to lose its milk- 

 teeth. 



Among the forms examined, probably only Ericulus and Echinops 

 possess the same number of functional mUk and permanent 

 antemolar teeth, but these forms have already a reduced dentition. 

 Of those provided with 44 teeth, viz. Oymmira and Talpa, we find 



in the former ' ' ^' reduced and functionless, while in Talpa, 

 omitting the 1st premolar, all the remaining milk-teeth are reduced 

 and though cutting the gum can hardly function (if at all) for 

 more than a week or two. 



The remaining genera examined shove this reduction in a varying 

 degree, the maximum being attained in Sorex, where in all 

 probability the entire milk series is reduced and functionless. 



If then it be a fact, as iS now generally believed, that the milk 

 dentition preponderates in the early Mammalia and in the living 

 Marsupials, then we must come to the conclusion that the living 

 Insectivora are specialized forms tending towards a Monophyodont 

 condition in which the preponderating dentition is the replacing 

 or permanent set. 



List of Eeferenoes. 



1. Bate, S.— "On the Dentition in the Mole." Trans. Odont. 



Soc. 1865-67, p. 261. 

 la. Brandt, E. — Ueber d. Zahnformel der Spitzmiiuse. St. 



Petersburg, 1878. 



