No. 377-] THE ORIGIN OF THE MAMMALIA. 329 
the orbit was open behind. He further observes the narrowing 
of the skull in front of the orbits and the bulbous aspect of 
the snout as a more definite character relating Tritylodon to 
the other theriodonts. As regards the enlarged front teeth, 
which have hitherto been considered as incisors, he thinks it is 
possible, since their roots ascend into the maxillary, that they 
may be canines. The skull further agrees with that of the 
theriodonts in the terminal position of the anterior nares and 
in the median anterior process of the premaxillary, which forms 
an internarial septum, also in the position of the posterior nares 
opening between the hinder molar teeth. The most charac- 
teristic region of the theriodont skull is that bordering the 
orbit, in which, unfortunately, the type specimen of Tritylodon 
is imperfectly preserved, so that it is impossible to determine 
positively whether there was a postorbital bar composed of the 
postorbitals (postfrontals, Seeley) as in the Theriodontia; the 
fossil shows an oblique fracture at this point, and the converg- 
ing plates, described by Owen as the parietal bones, are regarded 
by Seeley as the posterior processes of the postfrontal bones, 
because they are closely comparable to the similarly placed 
bones of the theriodonts. The prefrontal bone, on the other 
hand, appears distinctly as forming the antero-superior border 
of the orbit. “Hence,” Professor Seeley concludes, “ I believe 
that the remains of the skull go to show that Tritylodon was a 
reptile, and that the skull might be restored upon the theriodont 
plan.” In the same paper Professor Seeley figures pieces 
p. 1028) a portion of the lower jaw. 
Diademodon. 
This genus was founded by Seeley on the characters of the 
molar teeth (1895, 3, p. 1030, Pl. LXXXIX, Fig. 11). He 
describes the superior molars as wider than the inferior, with 
the crown low, subquadrate, or transversely oval. As pointed 
out by the writer in Science, these upper teeth are of extraor- 
dinary interest, since they show she typical tritubercular pattern. 
While the crown is roughly tubercular, the four prominent cusps 
correspond with the protocone, paracone, metacone, and hypo- 
