50 PACHYDERMATA. 



of the crown ridges in the last deciduous molar, and in the first 

 and second true molars in both jaws. 



Fig. 9 5 as previously described, represents a section of the last 

 upper molar of Mastodon Ohioticus. It consists of four prin- 

 cipal ridges, and a small talon lobe. The successively increasing 

 simplicity of form which has been traced from E. insignis, attains 

 its extreme limit in the molars of this species. The ridges are 

 transverse, terminating in a trenchant edge ; the ivory segments 

 are in regular angular lobes, the layer of enamel is of uniform 

 thickness, and the hollows between the ridges are very wide and 

 open, being almost rounded at the bottom. The cement is pre- 

 sent only in an exceedingly thin crust, continued over the fangs in 

 greater thickness. The common plane of the grinding ridges of 

 the crown is nearly horizontal, while it is more or less convex in 

 all the previously noticed species. It has not been deemed neces- 

 sary to give a delineation of the section of an inferior molar, 

 which differs in no respect from the upper, except in being com- 

 plicated with an additional ridge. 



To the same group belong two other species, M. angustidens 

 and M. Andium, and probably a third, M. Tapiroides, the den- 

 tition of which is but imperfectly known. The molars of the two 

 first differ from those of M. Ohioticus, in the same manner that 

 M. Arvernensis and M. Sivaiensis differ from M. latidens ; viz., 

 the crown ridges, instead of being transverse, are composed of 

 mammillae, which are placed more or less alternately, projecting 

 into the interspaces, and interrupting their continuity. The teeth 

 of M. Andium are remarkable in being invested with a coat of 

 cement, which fills up the bottom of the hollows, and is extended 

 over the mammillae in a considerably greater quantity than occurs 

 in any other species of true Mastodon. These three species, 

 M. Ohioticus, M. angustidens, and M. Andium, constitute a 

 distinct section, agreeing in having the three molars which precede 

 the last, viz., the third deciduous molar, and the penultimate and 

 antepenultimate true molars, uniformly characterized by having 

 their crown divided into three ridges in both jaws ; while the same 

 teeth in the preceding group have each a crown with four ridges. 



