THE OSTEOLOGY OF ELOTHERIUM. 285 
(£. imperator) from the Protoceras beds, the openings haye become minute; they are 
shifted laterally and haye no anterior grooves leading from them. 
The mandible is not the least curious part of this remarkable skull. The horizontal 
ramus is extremely long and nearly straight, with an almost horizontal inferior border. 
The depth and thickness of the ramus yary considerably; even in skulls of the same 
length the mandible is decidedly more slender in some specimens than in others. The 
materials are, however, not yet sufficient to determine whether this difference is of a spe- 
cific, sexual, or merely individual character. -A remarkable knob-like process is given 
off from the ventral border of the mandible, beneath p z, which is subject to much -yari- 
ation in shape and elongation, in accordance with the age and size of the animal. In 
young indiyiduals still retaining the milk-dentition, the process is a mere rugose eleva- 
tion, and in the adults of the smaller species it is hardly more than a knob, while in the 
large forms it becomes greatly elongated and club-shaped. No marked difference in this 
regard is observable between the species from the upper and those from the lower hori- 
zons of the White River formation, the process being relatively quite as long and promi- 
nent in Z. ingens from the Titanotherium beds, as in /. imperator from the Protoceras 
beds, but in the huge John Day species it has become particularly long and heavy. 
The symphysis is quite long and very thick and massive; the two rami are indis- 
tinguishably fused together and laterally expanded, so as to somewhat resemble the sym- 
physis of Hippopotamus, though not attaining any such extreme degree of massiveness as 
in the modern genus. The chin is abruptly truncated and flattened, and rises very 
steeply from below; on each side, beneath or a little behind the canine alveolus, there 
arises from the ventral border a second club-shaped process, similar to, but much heavier 
and more prominent than the posterior process already described. These two pairs of 
knobs give to the jaw a highly peculiar and characteristic appearance ; they form another 
of the enigmatical features of the Elotherium skull, for it is difficult to imagine what part 
they can have played in the economy of the animal. 
The two inferior dental series pursue a nearly parallel course, diverging backward 
but little, but behind the molars the two rami turn outward and diverge rapidly, so that 
posteriorly they are very widely separated, in correspondence with the great interval 
between the glenoid cavities of the two squamosals. The angle of the mandible is prom- 
inent and descends below the ventral border of the horizontal ramus, much as in Hippo- 
potamus, though not to the same extent. The ascending ramus is not high, but of con- 
siderable antero-posterior extent. The masseteric fossa is quite small, but very deeply 
impressed, and is situated quite high upon the side of the jaw. The condyle is relatively 
little raised above the level of the molar teeth, and it is sessile, hence inconspicuous, 
though it is large, transversely expanded, and strongly convex. The coronoid process 
