TOOTH-GENESIS IN THE CAVITD®. 281 
point at which the rotation of the cusps is presumed to have 
taken place. 
In tracing the course of the molar evolution in Cavia, it has 
been found that the tooth begins by the formation of a single 
cone, which subsequently becomes the antero-external cone of 
the adult tooth, so far agreeing with what has been noted in 
other mammalian orders. In the development of teeth from a 
multituberculate type as usually understood, one would not 
expect to find the development of a single cone taking place first, 
as is the case here and elsewhere, but of several. Ag I have 
already shown, this single cone of the guinea-pig has both 
external and internal cingula, the latter bemg the better marked. 
Both develope secondary cusps, which disappear in the course 
of the subsequent development of the tooth. There is thus a 
tendency to the suppression of cusps after a certain period, the 
adult tooth being less multituberculate than at an earlier stage, 
though more so than in its youngest condition. Consequently 
there is evidence in the later stages of development in favour 
of the Multitubercular theory. 
Though I have not personally met with any direct embryo- 
logical evidence in support of the Concrescence theory, yet upon 
general grounds I am disposed to accept it to a certain extent. 
This theory supposes a fusion to have taken place not only 
antero-posteriorly of teeth of the same dentition, but also 
transversely of teeth of different dentitions. The former would 
account for the diminution in the number of teeth of the same 
dentition in the transition from the Reptiles to the Mammals ; 
while the latter was suggested in order to explain the existence 
of the triple longitudinal rows of cusps as seen in the fossil 
Multituberculata. It is the latter part of this theory that I find 
myself at present unable to accept. 
The progressive shortening of the jaws would naturally tend 
to a crowding of the teeth, which may be conceived to have 
become fused antero-posteriorly as a result, and Ameghino [2] 
has adduced some presumptive evidence in support of this; and 
I have already referred to the fact that the cheek-teeth in the 
guinea-pig have similar anterior and posterior portions. It is 
possible to believe that there may be some close connection in 
the way of cause and effect between the two processes, though 
actual evidence is as yet wanting. On the other hand, it is 
