306 



ANATOMY OF VERTEBRATES. 



supported upon the common pulp, and do not become united 

 until the centripetal calcification has converted this into a common 

 dentinal base. Some anatomists have regarded the large incisor 

 so formed as an aggregate of two or three teeth ; but in Sorex 

 proper and Hydrosorex, the calcification of the lower incisor 

 spreads from a single point, and the interpretation of the notched 

 incisor of the Amphisorex, as the representative of these incisors, 

 might, by parity of reasoning, be applied to the human incisor 

 teeth, the dentated margins of which are likewise originally three 

 or four separate tubercles. 



The determination of the small teeth between the large an- 

 terior incisors and the multicuspid molars depends upon the 

 extent of the early anchylosed premaxillaries ; the incisors being 

 defined by their implantation in those bones, the succeeding small 

 and simple-crowned molars must be regarded as premolars, not 

 any of them having the development or office of a canine tooth ; 

 their homotypes in the lower jaw are implanted by two roots. 



The thickness of the enamel, in proportion to the body of 

 dentine, is unusually great in these small insectivores, and the 

 sharp points of the teeth long retain their fitness for the office 

 of cracking and crushing the hard or tough teguments of insects. 

 The enamel-pulp of the lower incisors is so large as to over- 

 lap, in the young Shrew, the growing margin of the socket, so 

 as to encase with enamel not only the crown of the tooth, but 

 also the contiguous part of the jawbone : the roots of these teeth 



also become anchylosed to 

 the jawbone, a reptilian cha- 

 racter offered by the Soricidce 

 alone in the Mammalian class. 

 In a large long-legged and 

 long-snouted African Shrew 

 (Rhynchocyon, Peters 1 ) the 

 lower incisors are bilobed ; 

 the upper ones absent, giving 

 the following dental for- 

 mula, fig. 242 : — 



242 



0.0 

 3^3' 



G" 



LI 

 iTT 



3.3 

 3.3 ! 



3^ 

 3^3 



= 34. 



The premaxillaries terminate 



Dentition of RUy >ajon, . Lmn '. ^ ft trencllant edentulous 



border, a, as in the true ruminant : to the hard gum covering it are 

 opposed the crowns of the six lower incisors, ib. B, i ; a canine, c 9 

 with a similar-sized but simple crown, seems part of the semi-cir- 



1 lxxxiv', p. 106. 



