18 BRITISH AND EUROPEAN FOSSIL MASTODONS 



North America and the Mammoth are so widely different in 

 the form of their molar teeth that they must be ranked 

 under distinct genera, the intermediate gradations are so 

 complete as to establish a passage from the one into the 

 other. Failing the characters of premolars and inferior in- 

 cisors, previously relied upon as distinctive of the Mastodons, 

 and abundant cement as distinctive of the Elephants, the 

 constancy of the ridge -formula in being isomerous (whether 

 ternary, quaternary, or quinary) in the intermediate molars, 

 appeared to furnish a sufficient technical demarcation be- 

 tween Mastodon and Elephas, and to subdivide the former 

 satisfactorily into the natural subgeneric groups of Trilo- 

 pJiodon and Tetralophodon. It remains to be seen whether 

 there is any intermediate species in which the characters of 

 these two groups are blended. 



Mastodon Sivalensis is regarded as having five ridges to 

 the ' intermediate molars,' instead of four ; but this remark- 

 able character being restricted at present to a single species, 

 it was deemed inexpedient to form a systematic section for 

 it alone, and it is ranged at the end of the Tetralophodons. 



Although a mesial, bipartient, longitudinal cleft along 

 the summit of the crown is very common in the molars of 

 most of the species of Mastodon, and usually absent in the 

 Elephants, there is one species of the former, M. (Triloph.) 

 Borsoni, in which the cleft is so obsolete, that Isaac Hays ' 

 founded the specific character upon the supposed absence of 

 this cleft. But the cleft, although but slightly pronounced, 

 is distinctly present in unworn germ-teeth of this form ; and 

 it is even visible in the original molar described by Abbe 

 Borson, upon which Dr. Hays relied for its absence. 



The plurality of the species in the first subgeneric group 

 of Elephas, namely Stegodon, are sufficiently distinguished 

 from the Mastodons by the higher numerical expression of 

 the crown-formula, in showing 7 or 8 ridges instead of 3 or 

 4 ; by the great quantity of laminated cement which fills the 

 transverse valleys ; by the ridges being convex, as in the 

 typical Elephants ; by the greater number of points to each 

 ridge ; and by the absence of a mesial dividing furrow. 

 But in one of the species, E. {Stegodon) Gliftii, there is an 

 obsolete indication of this furrow ; and its affinity to the 

 Mastodon is further evinced by the low or senary expression 

 of the ridge-formula. This species constitutes a frontier 

 form, through which the passage between the two genera is 

 effected ; but the details of the other dental characters show 

 that it is most nearly allied to the Stegodons, and the cha- 

 racters of the subgeneric group were constructed to admit of 



1 Transactions of the American Philosophical Society, ser. 2, vol. ir. p. 334. 



