56 PACHYDERM ATA. 



teeth in this species, to show where the deciduous series terminates 

 and the true molars begin. The last milk molar is followed 

 in antero-posterior succession, as in Dinotherium, by a tooth 

 which has its crown divided also into three ridges, and is thus 

 indicated to be the antepenultimate, or first true molar. It 

 measures four inches long, by about three in width, differing only 

 in size from the tooth which precedes it. The penultimate, or 

 second true molar (being the fifth in the order of succession), con- 

 sists, also, of three ridges, and measures about five inches by three 

 and a half. The third, or last true molar, consists of four prin- 

 cipal ridges, and a small heel ridge, which varies considerably in 

 amount of development. This tooth measures 7.25 inches or 

 upwards, by about 4.5 in width. 



The inferior true molars in M. Ohioticus, agree with the upper 

 in the form and division of their crowns, except the last which 

 has usually five principal ridges. They are narrower in proportion to 

 their length, and the subordinate talon ridges are more developed. 

 The molar formula in this species is, therefore, 3 milk molars 

 in the young animal ; and premol. + 3 premol. = 3 in each 

 side of both jaws of the adult ; the number of ridges in the dif- 

 ferent teeth according to their succession being 2 + 2 + 3 m tne 

 milk molars, and 3 , 3 7_ 5 in the true molars. With regard 

 to the number of teeth which are simultaneously present in the 

 jaw; the lower jaw of Tetracaulodon figured by Godman, 1 shows 

 the three milk molars in use, and the first true molar in its alveolus, 

 there being four out of the whole number of six teeth at one 

 time in the jaw. These are ultimately in advanced age reduced 

 to the last tooth, the others being shed. 



M. angustidens. — The dentition of M. angustidens is in- 

 volved in great confusion, in consequence of most authors, 

 who have written on this species, having mixed up, under this 

 name, two distinct forms, the one characterized like M. Ohio- 

 ticus, by a ternary, the other, by a quaternary division in the 

 ridges of the middle teeth. It will be necessary to enter 



1 Amer. Phil. Trans., New. Series, vol. iii. pi. xviii. 



