ELEPHANT AND MASTODON. 73 



the anterior ridges are first ground down into circular rings 

 of enamel enclosing a pit of ivory ; these rings then unite 

 into oval groups (PL VII. fig. 4 a, F.A.S.) ; 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 machwris 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 angu- 

 lar expansions in the middle of the ridge : such modifications 

 holding with great constancy in the different sj^ecies, and 

 yielding the characters by which they are most readily dis- 

 tinguished. The three constituent dental substances being 

 of unequal hardness, are worn unequally by the process of 

 trituration ; the hard enamel projects above the ivory, and the 

 softer cement wears more quickly than either. The grinding 

 plane of the tooth thus presents, throughout its period of 

 duration, a surface highly organized by natural inequalities, 

 to serve a constant tritiirating purpose. As the anterior 

 lamellae are worn down, the corresponding fangs are gradu- 

 ally absorbed. 



Between these extremes, furnished by the North American 

 mastodon and the existing Indian elephant, there is a series 

 of intermediate forms, which establishes an almost unbroken 

 passage from the one into the other. 



Fig. 1, PL IV. (fig. 6 a, PL II., P.A.S.^) represents a vertical 

 and longitudinal section of the last upper molar of an Indian 

 fossil species, which we have named Elephas insignis in this 

 work. It is selected as furnishing the best illustration of the 

 intermediate type of a proboscidean molar tooth, from which 

 those of the other species diverge in opposite directions. It 

 is in the most favourable state of age and use for showing 

 the characters, the four anterior ridges being affected by 

 wear, and the six posterior ridges entire, while the fangs are 

 fully developed, their mode of implantation in the jaw being 

 distinctly shown. The tooth is convex from back to front, 

 in the outline of the crown. The white mass in the centre 

 represents the body of ivory, which is projected upwards in 

 ten angular lobes terminating in a sharp edge. The height 

 of these lobes does not much exceed the width of their base, 

 and closely applied over them is seen a thick layer of enamel, 

 reflected up and down in a continuous zig-zag plate. The 

 interspaces of the five posterior ridges of enamel are com- 

 pletely filled up by a mass of cement, or ' cortical,' much 

 exceeding the enamel in thickness, and in quantity in nearly 

 as great an amount of development as the ivory core of the 



» See note, p. 79.— [Ed.] 



