237—56 
SIWALIK AND NAEBADA PEOBOSCIDIA. 
The ridge formula of the milk-molars is constant, and conforms to the normal 
tetralophodont order ; in the true molars, however, there is not an unfrequent ten- 
dency to assume a pentalophodont type, the hind- talon of many of these teeth not 
unfrequently, partly or entirely, taking the form of a fifth ridge. In the following 
ridge-formula these varieties are indicated : — 
Milk-molars. True molars. 
2 -f- 3 + 4 (4-5) + (4-5) + (5-6) 
2 + 3 H- 4 4 -f (4-5) + 5 
This tendency to the production of an additional ridge in the true molars of 
M. latidens will he subsequently shown to he a character which it possesses in 
common with M. sivalensis. This tendency to variation enables us easily to com- 
prehend how the passage from the tetralophodont Mastodons to the hexalophodont 
Stegodons {S. cliftii) was effected. A similar tendency to variation in the ridge 
formula of various species of European Mastodons is noticed by M. Gaudry.' With 
these very important variations in the molars, which, as in M. latidens, can be traced 
gradually from an incipient to a perfect ridge, it seems impossible that any naturalist 
can continue to maintain that there is at present no evidence of the passage of one 
species of animal into another. If the very small hind-talon of the five-ridged 
second upper molar of Mastodon latidens described above (Plate XXXVIII, fig. 1) 
were to develope into a true ridge, as we have seen to be the case with the talon 
in the four-ridged tooth {ihid. fig. 2), and were the median cleft to disappear 
and a little cement to appear in the valleys, the hypothetical six-ridged tooth would 
then be almost, if not quite, indistinguishable from the corresponding tooth of 
Stegodon cliftii. Such considerations appear to me to make it very doubtful whether 
the genus Stegodon should not be abolished and united with Mastodon. But here 
a new difficulty presents itself, since the genus Stegodon at one end presents 
elephantine characters in its molars, and at the other mastodontine. Thus S. cliftii 
(including S. sinensis) has the inner side of the upper molars the lowest, as in the 
Mastodons, while all the other Stegodons have the centre of each ridge the highest, as in 
the Loxodons and true Elephants. On the other hand, all the Stegodons have the 
elephantine character of having generally no median cleft to the molars, but this is 
a character which is equally shared with Mastodon horsoni and some other species of 
that genus. On the other hand again, the molars of Stegodon insignis are very close 
in general structure to those of Loxodon planifrons, and are certainly nearer to those 
of the true Elephants, than to those of the Mastodmis. Stegodon insignis, however, 
cannot be generically separated from S. cliftii, and we are therefore led to the con- 
clusion that if the latter species were united to the Mastodons, then the latter would 
have to be classed in the same genus as the true Elephants. There is, indeed, no real 
distinction between Elephants and Mastodons, although the species at either of the 
chain {Mastodon maximus and Euel&plias primigenius) are widely separated from 
Les Enctainements du Monde Animal.” Mammiferes Tertiaires, p. 181, Paris, 1878. 
