ELEPHANT AND MASTODON, 87 



met with in both jaws, one or two of the anterior teeth of 

 this set being suppressed. The last milk molar, above and 

 below, is three-ridged, while the penultimate has only two 

 ridges. These teeth are replaced vertically by an equal num- 

 ber of premolars, which represent the j)enultimate and last, 

 the two anterior teeth of this series being also suiopressed. 

 The last premolar, as well as the penultimate, is only two- 

 ridged, conforming to the ordinary rule of being simpler in 

 form than the milk molar which it succeeds. Of the three true 

 molars, the first or antepenultimate, in both jaws, is three- 

 ridged, repeating the complex form of the last milk molar, 

 while the penultimate and last are only two-ridged. This is a 

 very remarkable anomaly, of which no other example is 

 known among the Pachyderniata, as it is commonly the last 

 true molar which reiterates the form of the last milk molar. 

 The first true molar is protruded and in use before the last 

 molar is shed, so tkat in the adolescent animal there are two 

 contiguous teeth, which have each three ridges. We have, in 

 this circumstance, the first essential proboscidean character, 

 which at once distinguishes Dinotherium from the Tapirs and 

 allied genera, and indicates its near relations to Mastodon. 



In regard to the number of teeth which are in place and in 

 use at the same time, Dinotherium is less aberrant than 

 even Phacochoerij^, as the tvw premolars and three true 

 molars in the adult animal are simultaneously present in 

 both jaws. The molar formula in DinotheriuTn is therefore 

 2 premol. + 3 mol. = 5 in each side of both jaws ; and the 

 number of ridges in the different teeth, according to their 

 successive position in the jaw, is 2 -1- 8 in the milk molars ; 

 2 + 2 ill the premolars, and 3 -1- 2 4- 2 in the true molars. 



Mastodon. Sect. Trilophodon. — M. Ohioiicus. — 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 weU-known forms to Dinothe- 

 rium, there are three deciduous molars in both jaws, the 

 most anterior of the series being suppressed. Of these, the 

 antepenultimate, 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 (theoreti- 

 cally the third), measures 1-7 by 1*75 mclies. These teeth are 

 of the same form, each consisting of four poiuts, which are dis- 

 posed in two transverse ridges ; and they further correspond 

 with the same teeth in the ordinary Pachydermata by differ- 

 ing but slightly in relative size. The third milk molar, as in 

 Dinotherium, consists of three transverse ridges, each com- 

 posed of two pairs of confluent points. It measures three 

 inches in length, by 2*4 in width. The milk molars of tlie 



