250 • ON THE EXTINCT MAMMALIA OF 



The anterior four divisions of the crown are nearly of uniform size ; the others 

 undergo a successive reduction, and rather more so in the right than in the left tooth- 

 The former divisions of the crown are all worn so as to expose large tracts of dentine 

 bordered by thick festooned bands of enamel. In the anterior two divisions the den- 

 tinal tracts extend completely across, in the others they are confined to their respect- 

 ive lobes. The latter present the relative complexity of those of the teeth of 

 Mastodon Humholdtii. The exterior portion of the lobes are simply robust conical ; 

 their interior portion appears to be composed of a connate group of lobules or oifsets 

 from the main lobes. 



The fifth division of the crown presents a reduced and more simple condition com- 

 pared with those in advance, and the last division is composed of a transverse pair of 

 cones, of which the outer larger one, in the right tooth, is sub-divided. The posterior 

 two valleys contain the remains of a thin layer of cementum. 



Dr. Falconer has grouped all the Mastodons into two subgenera, Trilophodon and 

 Tetralophodon, based upon the intermediate teeth of the molar series possessing three 

 or four transverse divisions or ridges to the crown. The formulae he gives for the 

 ridged condition of the molar teeth in the two subgenera are as follow : 



1st molar. 2d. 3d. 4th. 5th. 6th. 



Trilophodon, ..123334 



Tetralophodon, ..234445 



The Mastodon americanus and the M. angustidens of Europe, according to the 

 formula, are placed by the author in the subgenus Trilophodon. The Mastodon 

 longirostris of Europe is placed in the subgenus TetralopJiodon. Of the two South 

 American species, M. and'mm and M. Humholdtii, the former was viewed as belonging 

 to Tetralophodon* the latter to Trilophodon; but more recently Dr. Falconer has 

 placed them both in the last-named subgenus.! 



According to the formula, Mastodon mirijicus would appear to belong to the sub- 

 genus Tetralophodo7i. 



The measurements of the lower jaw of M. mirijicus are as follow : 



Inches. 



Greatest breadth outside the position of the molars, and opposite the penul- 

 timate or fifth ridge of the crown, ..... 16i 

 Length from prolongation of chin to back of last molar, obliquely, . 17 

 Length from do. in the median line to the back of the last molars, . 16i 

 Length of the symphysial gutter from the posterior line of the symphysis, . 41 



Dr. Hayden observed many other remains of Mastodon, probably belonging to this 



* On the Species of Mastodon, etc. Quar. Jour. Geol. Soc. London, 1857, xiii, 307. 

 t On the American Fossil Elephant, etc. Nat. Uist. Review, London, 1863, 99. 



