ELEPHANT AND MASTODON. 33 



Elephants this cement substance completely fills up the hollows 

 between the plates of enamel. 



The production of the hard tootn 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 

 bodv of ivory which occupies the central mass of the tooth. 

 Simultaneously with this production of the ivory, a similar process 

 of solidification goes on in the corresponding and contiguous por- 

 tions 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 con- 

 tiguous 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 Mas- 

 todons, while they are numerous, slender, and confluent in the 

 Elephants. 



The three constituent dental substances are structurally dis- 

 tinguished by very different characters, and their combined modi- 

 fications 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 illustra- 

 tions of the opposite extremes of form presented by these teeth 

 in the Elephantidae. Taking the last tooth of the upper jaw as 

 the example ; in the former, the crown is nearly rectangular in 



D 



