THE OSTEOLOGY OF ELOTHERIUM. 289 
margin becomes much thicker and more rugose. The vertebrarterial canal, which is 
notably small, occupies much the same position as in Sus, opening posteriorly upon the 
dorsal side of the hinder border. The anterior extension of the transverse processes 
has conyerted into foramina (atlanteo-diapophysial) the notches for the inferior branches 
of the first pair of spinal nerves. On the ventral face of each process is a large fossa, 
enclosed between the side of the inferior arch and the greatly thickened posterior border 
of the process. The resemblance in shape to the atlas of Anoplotherium, to which atten- 
tion has already been called, affects more particularly the form of the transverse processes 
but they are more extended transversely than in that genus and are not so pointed at the 
postero-external angles. 
The avis (Pl. XVIII, Fig. 4) is a short, but very massively constructed bone, 
which in general shape and appearance resembles that of Lippopotamus. The centrum is 
short, anteriorly yery broad and depressed, but thickening posteriorly, and with a nearly 
circular and slightly concave hinder face. A strong and prominent keel runs along the 
ventral face of the centrum, enlarging backward, and terminating behind in a trifid 
hypapophysis. The odontoid process is short, heavy and conical, with no tendency what- 
ever to assume the depressed and flattened shape which occurs in so many White River 
ungulates. The yentral articular surface of the odontoid seems like something super- 
added to the process itself, for it is clearly demarcated by a groove running all around it, 
and projects slightly in front of the body of the process. On-the dorsal side of the 
centrum a broad and well-defined ridge runs backward from the odontoid along the floor 
of the neural canal. The atlanteal articular surfaces are yery broad and low, not rising 
so as to enclose any part of the neural canal. They are very oblique with reference to 
the median line of the centrum, with which they form angles of about 45°. These 
surfaces are slightly convex in both directions, and ventrally they project much below 
the level of the centrum. 
The transverse processes are short, thin and compressed, much less massive and 
widely extended than in Hippopotamus ; they are perforated by very large foramina 
for the vertebral arteries. The pedicels of the neural arch are low and short, but very 
heavy ; they are not pierced for the passage of the second pair of spinal nerves, as they 
are in Hippopotamus and in some of the pigs. The neural canal is decidedly small, 
especially its anterior opening; behind, it enlarges somewhat, particularly in the dorso- 
ventral dimension, the posterior opening being high and narrow, while in Hippopotamus 
it is low and broad. The neural spine is a large plate which is very thin in front, but 
becomes thick and massive behind, ending in a broad rugosity. This spine resembles 
that of Hippopotamus, but is not produced so far backward and does not overhang ‘the 
third cervical, The pestzygapophyses are large, slightly concaye, and present obliquely 
