449 Prof. LE. D. Cope—On the Proboscidea. 
forms of the family, but in the genus Hlephas they have disappeared. 
In all the species they are shed early in life in order to make way 
for the true molars. As the latter teeth are very large, and the fore 
and aft extent of the jaw is small, there is only space for one 
or two of them at a time. In most of the species the last molar so 
much exceeds the others in size, that it occupies the entire jaw, and 
the other molars are shed in order to accommodate it. In the genera 
Tetrabelodon, Dibelodon and Mastodon, the last premolar, and the 
first and second true molars are isomerous, i.e. have the same 
number of cross-crests. In Emmenodon and Elephas they are hetero- 
merous; that is, the number of cross-crests successively increases 
from front to rear. Thus in the three genera named the ridge 
formula is: P.M. 2-2-8; M, 3-8-4 and P.M. ?-?—4; M. 4-4-5 or 
4-5-6. In Hmmenodon the ridge formula is P.M. ?-?-?-5; M. 
6—~7—6—7-8; and P.M. ?—6-7; M. 7—8-9—10-12. In Elephas the 
formula extends from M. 6—6—7—8-9, to M. 9-15—14-16—18-27. 
Hach genus then has a certain range of variation in the number of 
molar crests, extending from a smaller to a larger number. This 
successive increase in complexity has been regarded by Falconer as 
the index to the successive evolution of the species, and rightly so. 
As already remarked, however, other measures of the same succes- 
sion cannot be overlooked, especially as the ridge formula changes 
in so gradual a manner as to render it unavailable as a basis of exact 
divisions, as has been remarked already by Lydekker. It is evident 
that the primitive Proboscidea had incisor teeth in both jaws, and 
that these had more or less of the usual enamel investment. The 
gradual modifications of these features is therefore another indication 
of the line of descent of these animals. The primitive Proboscidea 
had likewise four premolars, as is now seen in Dinotherium. The 
successive loss of these teeth is no less an index of the evolution of 
the modern types of the order, than the other modifications referred 
to. In general, then, the phylogeny of the order may be represented 
thus : 
Elephas 
Emmenodon 
| 
Mastodon Dibelodon 
Tetrabelodon 
Dinotherium 
Primitive Proboscidea. 
Within each genus certain parallel modifications of the composi- 
tion of the crowns of the molar teeth may be observed. ‘The cross- 
crests may be single, or they may be divided up into tubercles. 
The valleys between them may be open, (1) or they may be blocked 
by (2) a system of single intermediate tubercles ; (3) by numerous 
intermediate tubercles; or (4) by the thickening of the primary 
tubercles. I arrange the species according to these characters. 
v: b, 
scat aia alga 
