Williston.] Mosasaurs. 137 



at the inferior part of the bone is smallest, looking nearly in- 

 wards. That for the condyle is flattened, looks nearly forward, 

 and is vertical in position. The superior facet, oblique in posi- 

 tion vertically and transversely, is for union with the axis, and 

 is more or less confluent with the internal triangular surface 

 for the odontoid. On the outer side the bone is strongly convex 

 from above downward, with a flattened process on the posterior 

 upper part, directed upwards and forwards, and expanded 

 obliquely at its distal extremity. The inner side of the top 

 margin is beveled and striated, and is probably contiguous or 

 approximated with its fellow of the opposite side. Directed 

 downwards and backwards from the posterior inferior side, 

 there is a stout styliform process. Above it there is a small 

 tubercular process on the posterior margin of the upper process, 

 directed backward. 



In Platecarpus and Tylosaurus the intercentrum resembles the 

 same bone in Clidastes, being a little broader antero-posteriorly. 

 The lateral processes have the upper process somewhat broader, 

 and the styliform process is very much shorter. 



Axis. 



Clidastes velox. The neural spine of the axis is elongated 

 antero-posteriorly, its length above equal to that of its proper 

 centrum. It is thin on the anterior portion, the border sloping 

 upward, the posterior border thickened, stout, and oblique in 

 position. The transverse processes are flattened and horizontal, 

 the smallest of the costiferous series, and with only a small ar- 

 ticular facet for the rib. The stout postzygapophyses have their 

 faces looking obliquely ventrad and laterad. The zygosphenal 

 articular surfaces are Avholly wanting on the inner side of the 

 triangular cavity between the borders leading to the two articu- 

 lations. The articular surface of the centrum behind is smoothly 

 convex, a little broader from side to side than from above down- 

 ward, and only faintly emarginate above. The hypapophysis 

 is the largest of the series, and is suturally united with the stout 

 exogenous process of the centrum, which is directed almost di- 

 rectly ventrad, its posterior margin close to the articular bor- 



