MAMMALS. 575 



the crown is covered entirely and uniformly by a layer of enamel. 

 In other mammals the enamel penetrates into the dentine, and thus 

 forms folds (dens complicatus s. semicompositus) , and in some again 

 these folds extend so far towards the root of the teeth as to divide 

 them into different laminae (denies compositi s. lamellosi}. The 

 molars of the elephant afford an example of the last kind. Since, 

 now, the act of mastication wears away the less hard dentine more 

 quickly than the enamel, projecting lines arise on the crowns of 

 the semi-complex and complex teeth, with furrows between them, 

 as may be readily seen in such teeth as the molars of the ox. The 

 plates of the complex teeth are held together by an intermediate 

 substance named cement (cementum s. crusta petrosa] ; it is formed 

 by ossification of the capsule, within which the germ of the tooth 

 exists, and which is here folded. This cement or bony substance 

 covers also the roots of the teeth in mammals. Enamel is not pre- 

 sent in the teeth of all mammals; dentine on the contrary and 

 cement exist in every tooth. The division of the teeth into incisors, 

 canines and molars is derived from human anatomy. These three 

 kinds do not always co-exist where teeth in general are present, 

 thus the rodents have no canine teeth ; the number also of the teeth 

 is very different in different mammals ; but on this subject we refer 

 to the characters of the genera in the Systematic Arrangement of 

 this class. The first teeth (deciduous or milk-teeth) are shed 

 by mammals 1 , as they are by man, after a certain time, to make 

 room for other permanent teeth 2 . 



1 That the hog forms no exception in this respect, as may be found stated by 

 ARISTOTELES, needs scarcely to be insisted on, although BUFFON repeats the mistake 

 (Hist. not. v. p. no). Compare on the teeth amongst others F. CUVIEB Des dents des 

 mammiferes, Paris, 1825, 8vo, E. OvfExOdontography, London, 1840 1845, sVols. 8vo, 

 and by the same the copious article Teeth in TODD'S Cyclopedia of Anat. and Phys-lol. 

 IV. 1852, pp. 864 935. On the microscopic structure of the teeth much is due to the 

 investigations of LEEUWENHOECK especially, which within the last few years have been 

 confirmed by modern observers, as PuRKiNJE (see FR^ENKEL de penitiori dentium, 

 humanorum structura observationes, Wratislaviae, 1835), RETZIUS (MiTcroskopisTca under- 

 solcningar b'fver Tandernes Structur; Kongel. Vet. ATcad. Handlingar, 1836, Stock- 

 holm), OWEN 1. 1. and others. 



z [The Incisors are those implanted in the intermaxillary (premaxittary OWEN) bone 

 and the corresponding teeth of the lower jaw, whatever their shape and size. "The 

 tooth in the maxillary bone which is situated at, or near to, the suture with the 

 premaxillary is the canine, as also is that tooth in the lower jaw which, in opposing it, 

 passes in front of its crown when the mouth is closed. The other teeth of the first set 

 are the deciduous molars; the teeth which displace and succeed them vertically are the 

 premolars ; the more posterior teeth, which are not displaced by vertical successors, 



