36 PACHYDERMATA. 



a large angle with the unworn plane of the crown, and in the 

 upper grinders it slopes from the inside outwards, being the reverse 

 of what takes place in the Mastodons. In the lower jaw, the 

 crown of the last molar is concave from behind forwards, and 

 convex across ; the grinding plates, especially towards the posterior 

 end, recline backwards, and the plane of wear, which is concave, 

 slopes from the outside inwards, bearing a reversed relation to 

 that of the upper jaw. The side of the jaw to which the teeth 

 belong is readily distinguished by these characters, and by the cir- 

 cumstance that the upper grinders are convex on the outer, and 

 concave on the inner side, the reverse taking place in the grinders 

 of the lower jaw. The last inferior molar attains a length of fifteen 

 inches, and presents occasionally as many as twenty-six or twenty- 

 seven constituent plates in the largest sized individuals of the Indian 

 Elephant. 



When the teeth come into use, the digitated summits of the 

 anterior ridges are first ground down into circular rings of enamel 

 enclosing a pit of ivory ; these rings then unite into oval groups 

 (pi. 7, fig. 4a); and as the wear descends below their point of 

 separation, the smaller discs disappear in a common transverse 

 band which is bounded by a projecting edge, or rnachctris of 

 enamel. These edges, which represent, a transverse section of the 

 enamel plates, either run across in straight and parallel lines, or 

 they are minutely crimped and undulated, or dilated into round 

 loops, or angular expansions in the middle of the ridge : such 

 modifications holding with great constancy in the different species, 

 and yielding the characters by which they are most readily distin- 

 guished. The three constituent dental substances being of un- 

 equal hardness, are worn unequally by the process of trituration ; 

 the hard enamel projects above the ivory, and the softer cement 

 wears quicker than either. The grinding plane of the tooth thus 

 presents, throughout its period of duration, a surface highly 

 organised by natural inequalities, to serve a constant triturating 

 purpose. As the anterior lamellae are worn down, the correspond- 

 ing fangs are gradually absorbed. 



Between these extremes, furnished by the North American 



