70 FAUNA ANTIQUA SIVALENSIS. 



face constitutes tlie ' enamel,' or vitreous shell, wliicli encloses 

 the ivory core of the crown of the teeth, termiuating abruptly 

 where the fangs commence. 



Exterior to, and following the folds of this ' enamel pulp,' 

 there is another modification of the internal membrane of the 

 sac, the ossification of which, during the last stage of the 

 development of the tooth, constitutes the external ' cortical 

 layer,' or ' cement,' which, in a crust of greater or less thick- 

 ness, is continued over the enamel of the crown and down 

 upon the fangs. In the true elephants this cement substance 

 completely fills up the hollows between the plates of enamel. 



The production of the hard tooth takes place by a process 

 of calcification, which commences in the summits of the 

 ' ivory pulp ' segments, the solidification extending gradually 

 downwards along the digital processes, which unite into a 

 transverse plate ; and these plates, at their base, are combined 

 so as to form the common body of ivory which occupies the 

 central mass of the tooth. SimultaneoiTsly with this produc- 

 tion of the ivory, a similar process of solidification goes on in 

 the corresponding and continuous portions of the ' enamel 

 pulp,' forming a shell of enamel which is closely applied to, 

 and moulded on the form of, the ivory segments and their 

 digital subdivisions. When the calcification has reached the 

 common base of ivory, the enamel plates covering the conti- 

 guous segments of ivory unite along their lines of junction in 

 the bottom of the clefts between the ridges. 



The basal mass of the ' pulp nucleus ' is not connected in 

 a continuous surface with the bottom of the sac, but, as it 

 were, by pedicles which, after the solidification of the body of 

 the tooth, elongate and become contracted, with more or less 

 of subdivision. These pedicles undergo the same process of 

 calcification, and form the fangs by which the tooth is 

 implanted in the jaws. The fangs bear a relation to the 

 divisions and vertical height of the crown, being few, thick, 

 and more or less distinct in the mastodons, while they are 

 numerous, slender, and confluent m the elephants. 



The three constituent dental substances are structurally 

 distinguished by very different characters, and their combined 

 modifications in the molar teeth furnish the best differential 

 marks for the arrangement of the groups of Mastodon and 

 Elephas, and for the discrimination of the different species. 



The molars of the North American mastodon and of the 

 existing Indian elephant may be selected as convenient 

 illustrations of the opposite extremes of form, presented by 

 these teeth in the ElephantidEe. Taking the last tooth of the 

 upper jaw as the example ; in the former, the crown is nearly 

 rectangular in outline, somewhat higher in front than behind, 



