328 THE AMERICAN NATURALIST. (VoL. XXXII. 
other tubercles of the crown. It is important to direct atten- 
tion to this divergence of the dental series posteriorly, which 
shows that these animals are not typical multituberculates like 
Tritylodon, in which animal the dental series are parallel with 
each other as an adaptation to the forward and backward motion 
of the jaws. The lower jaws are formed, as in the theriodonts, 
by rami which are coalesced at the symphyses, consisting 
externally of dentary bones which are produced posteriorly into 
a high coronoid process, and exhibit also an inferior posterior 
angle, a character which is entirely wanting in the Eocene 
Multituberculata. As in the cynodonts and typical reptiles, the 
jaws unite with the skull by elongate articular bones. 
So far as known (1895), there are no fundamental differences 
in the skeleton to separate the Gomphodontia from the Cyno- 
dontia, and these two groups are regarded by Seeley as related 
in the same way as are groups of marsupials with similarly 
differing dentition. The skeleton doubtfully referred to Micro- 
gomphodon shows a distal transverse expansion of the ribs into 
triangular extremities, as in. Cynognathus, so as to form an 
interlocking union similar to that of the zygapophyses on the 
neural arch. In the same skeleton the pelvis resembles that of 
the Cynodontia, except in the apparent exclusion of the pubic 
bone from the acetabulum. For in the Cynodontia the pubis- 
takes its normal part in forming the acetabulum. The long, 
lateral trochanter of the femur is less developed than in the 
Cynodontia. In the tarsus the astragalus and calcaneum are 
both large bones, but the calcaneum exhibits no tuber calcis. 
The scapula is constructed on the same plan as in Cynognathus. 
The same is true of the humerus. The interclavicle is thin, 
wide, and long. 
Tritylodon. 
The skull of Tritylodon longevus, described by Owen in 
1884, was placed hesitatingly with the Jurassic and Eocene 
Multituberculata until reéxamined by Seeley in 1892 (1895, 2, 
p. 1025). He refers the skull to the Lady Frere level, and 
finds some evidence that the orbit was closed posteriorly, as 
among the theriodonts, Professor Owen having assumed that 
