THE OSTEOLOGY OF ELOTHEEITJM. - v .f 



margin becomes much thicker and more rugose. The vertebrarterial . canal, which is 

 notably small, occupies much the same jsosition as in" $us, opening posteriorly upon the 

 dorsal side of the hinder border. The anterior extension of the transverse processes 

 has converted 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 axis (PL XVIII, Fig. 4) is a short, but very massively constructed bone, 

 which in general shape and appearance resembles that of hippopotamus. The centrum is 

 short, anteriorly very 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 ventral 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 very 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 pcstzygajTOphyses are large, slightly concave, and present obliquely 



