PHENACODUS. 
175 
exterior base. The narrowing of the posterior part of the crown of this 
tooth is a peculiar feature, reversing the proportions frequently seen in teeth 
in the same position. It corresj)oncls in this respect with the last inferior 
premolar, and was found in place as described. 
The last premolar is subround in the outline of the base of the crown. 
My notes, taken at the time of discovery, state that it supported four tuber¬ 
cles. A fragment has been lost from the anterior part of the crown, so that 
a tubercle rfiay be missing; but the remaining and principal part of the 
crown now supports four tubercles of unequal sizes. There is, on the outer 
side, a prominent one over the posterior root, and a much smaller and lower 
one in front of it. On the inner part of the crown, there is a stout and 
prominent tubercle, opposite the notch between the two external ones, and 
separated from the latter by a deep longitudinal valley. At the posterior 
part of this valley, near the very convex posterior border of the crown, is 
a low tubercle. There is a trace of a cingulum on the external base of the 
croAvn, and a cingular tubercle at the posterior low tubercle just described. 
The first true molar differs in its form from the second and third, 
which nearly resemble each other. The crowns of the latter consist of two 
external and two internal Ioav, conic tubercles, Avhich stand considerably 
within the base on both sides. A lower tubercle stands between each pair 
of the anterior, exterior, and posterior sides, but none between the interior. 
The croAvn of the first molar is considerably broader than long, and sup¬ 
ports five low, conic tubercles. One of these is on the posterior external 
angle; one is within the inner extremity; two are on the anterior border, 
and one on the posterior border of the crown. Besides these, there is a 
strong cingulum round the inner extremity of the crown, Avhich rises almost 
as high as the inner tubercle described. There is a tubercular cingulum 
along the posterior border, and one round the ba^e of the outer anterior 
tubercle, but none on the external base of the crown. 
Besides the characters already assigned to the last two molars, it may 
be added that they possess a basal ledge all round, which is in some places 
weak; in others, especially at the posterior outer angle, quite prominent. 
The enamel of all the molars is, where not worn by mastication, slightly, 
wrinkled. 
