PORTLAND STONE. 9 
the cervical vertebrae, is confirmed by the structure of the bony framework of the 
paddles. The modification in question, like the fore-and-aft compression or shortness of 
the cervical centrums, exemplifies the nearer resemblance, I will not say affinity, of 
Pliosaurus to Ichthyosaurus ; the segment of the natatory limb which answers to the anti- 
brachium and the cnemion in the higher Vertebrates being scarcely more marked or 
differentiated in the present genus of huge Sawropterygia than in Ichythyopterygia. 
The first indication of the modification in question was given by a specimen in which 
only the proximal halves of the two bones, or two chief bones, succeeding the femur were 
preserved along with that bone. 
The inference which I drew from close inspection and comparison of the preserved 
portions of the two cnemial bones was subsequently confirmed or strengthened by the 
condition of the same segment of the fin-bones in the magnificent specimen of those 
bones restored by Mr. Mansel, probably from the bones of the skeleton of the Pliosaurus 
grandis to which the above described skull belonged, and of which fin-bones a cast is 
exhibited in the Paleontological Gallery of the British Museum. 
Nevertheless, with the close general affinities illustrated by most of the framework and 
dentition of Plio- to Plesio-sawrus, | waited in hopes of an opportunity of acquiring 
certainty as to the structure of the middle segments of the limb before committing 
myself to a publication of what I am now able to positively state to be a generic 
character of Pliosaurus. 
The wished-for evidence reached me this year in the form of a block of Portland stone, 
in which were imbedded the femur, cnemion, tarsus, and part of the metatarsus and digits 
of a right hind-limb, referable by the character about to be described to the genus Plio- 
saurus. ‘The specimen, moreover, had the additional interest of being the first evidence of 
that genus from the Upper Oolite of Portland Island. It is figured rather less than 
half the natural size in Pl. IV. 
The femur (Pl. 1V, figs. 1 and 2, 65) presents the usual plesiosauroid proportions 
and characters, the pliosaurian affinity beg faintly indicated, as usual, by the greater 
extent of the tract (above 67’) external to the fibular division (above 67) of the 
distal articular extremity: the tuberosity (fig. 2, ¢-) and contiguous rough surfaces 
for the attachment of muscles, at and near to the proximal end, are also a little 
more strongly marked, as in the Pliosaurus grandis, but the tuberosity is less 
distinctly prominent than in Pliosaurus trochanterius. The head of the femur 
(fig. 3, nat. size), subconvex and cblong, is slightly nipped in, as it were, near its outer 
third part from side to side; the long axis of this surface is at right angles to the plane of 
the expanded and compressed distal end of the bone. A few of the crateriform eleva- 
tions on the rough articular surface are preserved. The inner side of the bone is exposed 
in the block of matrix. The roughness for ligamentous or tendinal attachment ceases 
about one third of the way down the shaft. This part, gradually contracting, assumes 
first a circular transverse section, then becomes compressed from without inwards instead 
2 
