THE TEETH OF MAMMALS. 291 



from the human tooth, as being familiar to us all, a quadrate 

 crown, with four cusps at its corners, is common to many 

 animals ; the oblique ridge, present in some apes, not in 

 others, is met with also in some Insectivora, e.g. the Hedge- 

 hog. Around the neck of many teeth runs a pronounced 

 ridge, the " cingulum," and this may be produced up into 

 additional cusps. 



A very instructive series of comparisons of the molar 

 teeth of Insectivora has been made by Mr. Mivart (Journal 

 of Anat. and Physiol. : 1868); pointing out that within 

 the limits of this group a great variety of patterns is met 

 with, the several modifications being connected by transi- 

 tional forms. 



It would appear that upon the molar teeth (upper) of 

 Insectivora there are four principal cusps (lettered a, b, c, d, 

 in the figure) which are more or less connected by ridges ; 

 such simple teeth are met with in the elephant mice 

 (Macroscelides), and hedgehog. The cingulum is well 

 developed in most of the group, and the further complexity 

 of the crowns, which often bristle with sharp points, is 

 brought about by the elevation of the cingulum into long 

 sharp points, equalling, or exceeding in length, the principal 

 cusps of the tooth. 



Thus in Urotrichus, a Japanese creature having affinities 

 with the mole, the external cingulum is elevated into three 

 distinct pointed cusps, united by ridges with the two prin- 

 cipal cusps, an arrangement which gives a sort of W pattern 

 to the surface, while to the inner side the cingulum forms 

 another cusp, so that there are in all eight cusps; the 

 common mole has the third cusp developed from the outer 

 cingulum, but its two inner principal cusps are fused together 

 and lose their distinctives. The suppression and fusion of 

 cusps is carried to a much greater extent in the compressed 

 teeth of the iridescent mole (Chrysochloris), but there are 

 intermediate forms which render it easy to identify its 



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