PTEEOSPHENUS SCHWEINPUETHI. 311 



is divided into two lateral pocliet-like cavities, separated by a considerable interval ; 

 the articular surfaces look upwards and inwards. Above these cavities the posterior 

 face of the neural arch is enormously expanded, concave from side to side, and produced 

 upwards and backwards into two long pointed processes (/.p.)i from the ends of which 

 a ridge runs downwards and forwards to the base of the anterior zygapophyses ; it is 

 this expansion of the posterior part of the arch and the presence of these remarkable 

 lateral processes that is especially characteristic of the genus, though the beginning 

 of a similar arrangement occurs in the allied and contemporary Palceoplds *. As in 

 that genus also the neural spine {n.sp.) is high, strongly compressed laterally, but it 

 seems to slope more backwards in the Egyptian form. Compared with the North- 

 American species, P. schucherti, described by Lucas f , this form seems to diflPer only 

 in having the posterior expansion of the neural arch considerably deeper, so that the 

 free part of the lateral processes is shorter. 



Among the vertebrae collected there is a considerable range of variation in size, but 

 very little in structure, the chief points of difference being in the relative length and 

 w^idth of the neural spine (see PL XXVI. figs. 5, 6) and the degree of prominence of 

 the lateral processes. In some also the anterior process of the hypapophysis is much 

 smaller than the posterior, and may in some cases be entirely wanting. 



The high neural spine and the downwardly directed articulation for the ribs seem 

 to show that in this snake the body was laterally compressed and more or less ribbon- 

 like, as in some recent aquatic snakes, in which the yertebrse are to some extent 

 similar. The occurrence of the remains in association with remains of Sirenians, 

 Zeuglodons, and Fishes lends further support to the belief that this species was aquatic 

 or semi-aquatic in its habits. 



C. 10194. Complete vertebra. Type specimen figured in Geol. Mag. [4] vol. viii. 1901, p. 439, 

 fig. 2 ; also on Ph XXVI. figs. 4, 4 a, 4 b. The dimensions (in centimetres) of tiiis 

 specimen are : — 



G-reatest height f i-om top of neural spine to end of hypapop'iysis . 8--5 



width (between ends of transverse processes) 



2-5 



Width of articular cup of centrum 1-6 



Height of articular cup of centrum 1-4 



Width of zygosphene 1-9 



Extreme length of centrum 31 



Width between the ends of the lateral processes of arch . . 4 app. > ■ 



E. 3;58. Similar, though rather smaller, vertebra. 



C. 10195. Smaller vertebra, with long slender spine and prominent lateral processes ; the anterior 

 end is somewhat abraded. Figured PI. XXVI. fig. 5. Extreme height fj'5 cm. ; 

 extreme length of centrum 2 cm. app. 



* Owen, EeptiUa of the London Clay, pt. iii. pi. xiii. figs. 5-7. 

 t Proc. U.S. National Museum, vol. ssi. (18U9) p. 637, pi. xlv. 



