INTRODUCTION. Ixxi 



marshes, and by a third in the sea, or on the banks of rivers. 

 The grinding surface of the molar tooth, for example, may for 

 this purpose be elevated into a pair of transverse ridges, and 

 we find such molars in the Kangaroo, the Tapir, and the Manatee, 

 as also in the extinct Diprotodon, Nototherium, and Dinothe- 

 rium. The small anterior molars of the Mastodon giganteus 

 likewise present this form. It would be difficult to select from 

 the Mammalian Class the constituents of a more heterogeneous 

 group than would be constituted by the character which M. 

 F. Cuvier has assigned as the true guide to the formation of 

 the most natural and uniform genera in Mammalogy. 



Even in regard to teeth adapted to carnivorous habits, were 

 these characters to form the sole guides in classification, species 

 of placental Mammalia would be associated with those of the 

 ovoviviparous sub-class ; and M. F. Cuvier, in illustrating his 

 generalization, observes : " Les sarigues, les p^rameles, et les 

 dasyures se sont reunis aux Insectivores, &c., &c., et je crois 

 avoir ete conduit a ces modifications par des motifs legitimes. '^1) 



The class of tissues in which teeth should rank, has fre- 

 quently been a subject of controversy in Systems of Histology ; 

 the fact being overlooked that they have not the same unity of 

 composition as hones or epidermal appendages. One constituent 

 of teeth, viz., the cement, unquestionably ought to rank with the 

 osseous tissue ; and the dentine, or ivory, which was described 

 for the last time in this country, in July, 1838, as being " like the 

 hair, arranged in concentric layers," (2) bears, on the contrary, a close 



(1) Loc. cit. p. xi. 



(2) Medico-chirurgical Review, p. 43. In France the dentine continued to be described, 

 as late as the end of 1839j as " compose de cones lamelleux extremement minces, s'emboitant 

 les lines les autres, &c." De Blainville, ' Osteographie' Fascicule premier, Primates, p. 14, 1839. 



