54 PACHYDERMATA, 



according to their successive position in the jaw, is 2 + 3 in the 

 milk molars; 2 + 2 in the premolars, and 3 + 2+2 in the true 

 molars. 



Mastodon. Sect. Trilophodon. — 31. Ohioticus. — The next 

 degree of deviation from the ordinary dental rule is presented by 

 Mastodon Ohioticus. In this species, which appears to be the 

 most nearly allied of the well-known forms to Dinotherium, 

 there are three deciduous molars in both jaws, the most 

 anterior of the series being suppressed. Of these, the antepenul- 

 timate, or ^anterior tooth (being theoretically the second) in the 

 upper jaw, measures 1.4 inches in length, by about 1.4 in width; 

 and the penultimate, or second (theoretically the third), measures 

 1.7 by 1.75 inches. These teeth are of the same form, each 

 consisting of four points, which are disposed in two transverse 

 ridges ; and they further correspond with the same teeth in the ordi- 

 nary Pachydermata by differing but slightly in relative size. The 

 third milk molar, as in Dinotherium, consists of three transverse 

 ridges, each composed of two pair of confluent points. It mea- 

 sures three inches in length, by 2.4 in width. The milk molars 

 of the lower jaw differ in no important respect from those of the 

 upper, except in being narrower in proportion to their length : and 

 in the greater development of the anterior and posterior subordinate 

 talon ridges. 



With respect to the premolars, the statements which have 

 been advanced regarding them are conflicting. They have 

 never been observed in either of the jaws by Godman, Hays, 

 Cooper, Harlan, or any other of the American Naturalists who 

 have described the dentition of M. Ohioticus; nor has their 

 presence been noticed by Dr. Grant. But Professor Owen, in 

 his British Fossil Mammalia, affirms that they have been recog- 

 nised in this species ; and, in his Odontography, 1 he figures and 

 describes a tooth as the penultimate premolar of the upper 

 jaw. It is there stated to be composed of two bifid transverse 

 ridges, girt by a basal cingulum, and to be of a simpler form than 

 the second deciduous molar ; the crown being broader in proportion 



' Loc. cit. p. 260. PI. 144, fig. 3, p. 1. 



