OF THE MESOZOIC MAMMALIA. 
245 
be called cusps. The Diplocynodon molar presents a decided advance upon that 
of Enneodon in the development of the paracone, which is much more pro- 
minent. In Ami)liithenum (text, fig.2), the paracone is subequal to the pro- 
tocone in several of the molars, and the heel is on the level of the internal 
cingulum, from which, according to Owen, there arise one or two small cusps.^ 
Internal cusps which develop in this manner are from the first separated from 
the external cusps by a longitudinal valley instead of being united with it by 
divergent ridges, and cannot therefore at any stage possess a sectorial blade, 
such as is more or less distinctly developed in the Spalacotherium and Slylacodon 
molar. 
5°. It follows also that the triangle of cusps presented by the Peraspalax molar 
cannot, with probability, be considered as representing a tritubercular stage and that 
the AmpliitlieriidcB furnish the key to the mode of derivation of the internal cusps of 
the molars of the Peralestidce. The inferior molars of Perasp)alax and Paurodon are 
apparently very similar (see PL VIII, fig. 9, m^, and fig. 9, text), consisting of a 
prominent external cone, and two internal cusps followed by a third cusp at the end 
of the crown. As pointed out in the synopsis of molar types, this internal surface 
strongly suggests the Dryolesies pattern, but may be clearly distinguished by the ab- 
sence of transverse ridges and the presence of a longitudinal valley between the cusps 
instead of a transverse valley opening inwards. The internal cusps have probably, 
therefore, arisen from the internal cingulum^ but these molars do not seem 
to be a later development of the Ampliitherium type because both the paracone 
and metacone are wanting, the main cone showing no trace of the lateral cusps 
upon its slopes. The superior molars of Peralestes, however, when viewed 
from above (Plate VIII, fig. 8), present one large internal and two smaller 
external cusps disposed in a triangle opening outwards, and as this is the 
general disposition of superior cusps of the tritubercular type, we must admit 
the possibility that the smaller cusps do represent the para and metacones in a 
stage of inward rotation not accompanied by the production of the sectorial 
blades, for this is by no means an essential feature of the tritubercular molar. 
The history of the derivation of the molars of the Peralestido} must, therefore, 
be left in some doubt; while the balance of evidence points to a line of development 
similar to that in progi'ess in the Amphitheriid(B, although the line of descent 
appears to be different. 
1 A8 previously stated, the writer has not personally examined the internal surfaces of the molars of 
this genus. 
»Nui 
important i 
of the post 
Allen, op. c 
molars are 
