PALiEOSYOPS LEIDY, AND ITS ALLIES. 
349 
premoliir of tliis jaw appears to be more developed than in T. aiKridens and the last 
premolar differs more from the third in size than in the smaller species of this genus. 
It is difficult to decide whether the last premolar in this species is really as 
complex as the first true molar, as its crown is much abraded, although the pos- 
terior inner angle of the same is not as much worn off as the rest of the tooth, which 
apjDears to lack any signs of an entoconid. If this is the correct interpretation 
none of the premolars in this species are as complex in their structure as the 
true molars. In the best preserved jaw the last inferior molar has a very high 
crown with a long antero-posteiior diameter. The posterior tubercle of the last 
inferior molar is large and laterally placed ; its anterior crest was strongly developed, 
the posterior one being much less so. The large size of this jaw and the peculiar 
characters of its sjmiphysis, would naturally lead us to compare it with the jaw of 
Diplacodon. The isolation of the first premolar in T. hyognathtis is a character not 
found in Diplacodon, in which the first premolar is placed close to the second, a 
long single diastema between the former and the canine intervening. The length of the 
symphysis in T. Jiyognatlms is probably greater compared with the size of the jaw 
than in Diplacodon, (see table of comparative measurements.) The anterior portion 
of the jaw in Diplacodon is not nearly so horizontal in position as in T. hyognathus. 
The wide spreading canine and nearly straight outline of the incisors correspond 
very closely in character to those of Diplacodon. 
Mandible. — The jaw of T. hyognatJms is much elongated and narrow. The 
region of the symphysis is very Suilline in character ; that is to say, anterior to 
premolar 2 the jaw is more nearly horizontal than in the other species of this 
subfamily, and this portion forms a much larger angle with the posterior two-thirds 
of the horizontal ramus than in the other species. The horizontal ramus differs 
from that of Stis in the fact that it is more slender, and in the region below the second 
premolar being only about one half the depth that it is below the last molar, whereas 
in Stis these vertical diameters are about equal. The posterior border of the ramus 
is straight and presents no inflection and posterior concavity so characteristic of the 
jaw of P. paliidosus. The region of the angle is very broad, thin, and strongly 
everted. The inferior border of tbe angular portion of the jaw is turned inward. 
The coronoid portion is destroyed. The alveolar border is straight, and a considera- 
ble space intervenes, between the tubercle of the last inferior molar and the 
ascending portion of the ramus. The characters of the symphysis of the jaw are 
striking. The symphysis is much more elongated in T. hyognathus than in any 
other species of this subfamily ; its posterior limit is only slightly more extended 
than in other species. The symphj'sis commences at the middle of premolar 3, but 
the anterior portion in front of premolar 2 is extremely elongated and narrow. 
Between the second premolar and the canine the jaw is very much com- 
pressed ; so much so, that when viewed from above there is a striking difference 
between the transverse diameter of this portion, and that of the middle region of 
the jaw. The narrowest part of the mandible is just in front of the second pre- 
