320 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 8, 



of new forms. It would be foreign to the main object of the present 

 communication, and beyond the limits within which it is necessarily 

 restricted, to discuss in detail the grounds on which the arrangement 

 is founded. As this will be done more fully elsewhere, I shall content 

 myself here with stating them in a general way, and with indicating 

 where the assailable points are. Although the Mastodon of 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 incisors, 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, quaternar}'^, or quinary in the intermediate molars, appeared 

 to furnish a sufficient technical demarcation between Mastodon and 

 Elephas, and to subdivide the former satisfactorily into the natural 

 subgeneric groups of Trilophodon 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 " in- 

 termediate molars," instead of four ; but this remarkable character, 

 being restricted at present to a single species, it was deemed inex- 

 pedient 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 spe- 

 cies 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 )Cliftii, there 

 is an obsolete indication of this furrow ; and its affinity to the Mas- 

 todon 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 characters of the subgeneric group were con- 

 structed to admit of its reception among them. Two of the Loxodons, 

 namely E. (Lox.) planifrons and E. {Lox.) Africanus, have a ridge- 



* Transactions of the American Philosophical Society, ser. 2, vol. iv. p. 334. 



