1893.] The Titanotherium Beds, 215 
was attended by a very marked development in certain por- 
tions of the skeleton, noticeable among which are the follow- 
ing: A variation in shape and an increase in the size and 
length of the horncores as compared with the size of the skulls, 
attended, near the summit of the beds at least, by a decided 
shortening of the nasals. 
There were also changes taking place in the dentition of 
these animals, especially in the number of incisors and in the 
structure of the last, upper, true molar. The number of inci- 
sors, though probably never constant, even in the same spe- 
cies, shows a tendency to decrease in skulls found near the 
summit of the beds. At the base of the beds the number of 
incisors is from one to three on aside, while at the top there 
‘are never more than two on a side, often only one, sometimes 
none. In skulls from the very lowest beds the incisors have 
already become so rudimentary as to be no longer functional. 
As would be expected, the number of incisors decreased after 
they became of no functional value. In the matter of incisors 
the Titanotheriidse were, at the time of their extermination, in 
a fair way to accomplish just what the somewhat related but 
more persistent Rhinocerotide have very nearly succeeded in 
doing, viz., the elimination of the incisor dentition. The 
number of incisors in the Titanotheriide varies with age in 
the same individual. These teeth were but loosely set in the 
comparatively thin alveolar border between the canines, and 
showed a decided tendency to drop out in old age, their alve- 
oles afterward becoming entirely closed. This is well shown 
in several instances where on one side of a jaw there may be 
1, 2, or 3 incisors, while on the other side of the same jaw 
there will be 0, 1, or 2 and no alveole for the missing tooth. 
The number of incisors can hardly be considered as of either 
generic or specific importance in the Titanotheride where 
they are no longer functional and vary with individuals in 
the same species and with age in the same individual. The 
same may be said of the presence or absence of the first pre- 
molar. 
In the structure of the last upper true molar we find a 
change taking place in the development of a posterior, inner 
