PTEEOSPJIENUS SClIWElNEUllTIir. 
311 
is divided into two lateral pocket-like cavities, separated by a considerable interval; 
tile articular surfaces look upwards and inwards. Above these cavities tlie posterior 
face of the neural arch is enormously expanded, concave from side to side, and produced 
upwards and backwards into two long pointed processes {l-p.), 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 Falceophis * . As in 
that genus also the neural spine (n.sj).) is high, strongly compressed laterally, but it 
seems to slope more backwards in the Egyptian form. Compared with the North- 
American species, P. schucherfi, described by Lucas f, this form seems to differ 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 vertebrm 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 
width of the neural spine (see PI. XXVI. figs. 5, 6) and the degree of prominence of 
the lateral processes. In some also the anterior process of the hypapophysis is mnch 
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 vertebrae 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 PI. XXVI. figs. 4, 4 a, 4 b. The dimensions (in centimetres) of this 
specimen are : — 
Greatest height from top of neural spine to end of hypapopliysis . 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 
Widtli of zygosphene 1‘9 
Extreme length of centrum 31 
Width between the ends of the lateral processes of arch . . 4 app. 
R. 3358. 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 6‘5 cm. ; 
extreme length of centrum 2 cm. app. 
* Owen, lieptilia of the London Clay, pt. iii. pi. xiii. figs. 5-7. 
t Proc. U.S. National Museum, vol. xxi. (1899) p. 637, pi. xlv. 
