398 CRETACEOUS REPTILES. 
anchylosed neurapophyses. The articulated cervical ribs in the Pl. pachyomus 
have a similar low position on the centrum. 
The under part of the centrum presents two deep pits from which the vascular 
canals ascend, divided by a moderately thick convex longitudinal bar (fig. 4). 
The non-articular surface of the centrum is smooth, and the sides of the 
centrum slightly concave. 
A very interesting and well-marked additional species of the singular genus 
Plesiosaurus is thus indicated by the present unusually perfect fossil vertebra. 
As it was discovered on one of the estates of his Grace the Duke of Norfolk, J 
avail myself of the opportunity of fulfilling a request of my lamented friend Mr. 
Dixon, and of gratifying my own wishes, by dedicating this new species to the 
memory of Lorp Bernarp Howarp, a young nobleman of great promise and 
most amiable disposition, and who had given much attention to the science of 
geology: he died suddenly in Egypt at the early age of twenty-one years, whilst 
pursuing his travels in order to acquire a knowledge of the antiquities, the arts, 
and policy of distant countries. 
Plesiosaurus constrictus, Owen. (Tab. XX XVII. figs. 6 & 7.) 
The species of Plesiosaurus from the chalk-pit at Steyning, Sussex, indicated by 
the centrum of a middle cervical vertebra, which is figured in Tab. XX XVII. figs. 6 
& 7, differs from that of the Plesiosaurus Bernardi, Tab. VI., in its great length 
as compared with the height and breadth of the articular surfaces of the centrum, 
and in the small size of the costal articulation, the pleurapophyses having been 
unanchylosed to the centrum ; it also differs from all the species of Plesiosaur 
hitherto defined in the degree of lateral constriction of the centrum between 
those surfaces, if this be natural. The free or non-articular surface of the 
centrum is rugose, showing the coarsely fibrous texture of the bone. The 
under surface is slightly concave, both transversely and longitudinally, is 
subquadrate and oblong, with two approximated vascular orifices at its centre, 
separated by a slight rising, which is not developed into a ridge. The small 
costal surfaces, pl, are elliptic, situated at the middle of the ridge dividing the 
under from the lateral surfaces of the centrum; twice their own vertical 
diameter below the neurapophysial surfaces, and equidistant from the two ends 
of the centrum. The articular surfaces here are slightly convex at their 
circumference, slightly concave in the rest of their extent, with a feeble 
longitudinal rising at the centre interrupted by a transverse linear groove. 
