516 Richard Owen, Esq., on the 



ingham : but after a comparison of Hawkins's Plesiosaur with the cast of the 

 Plesiosaurus Dolichodeirus in the collection of the Geological Society, and 

 with the original remains of a second specimen of the same species in the 

 British Museum, figured in Dr. Buckland's comprehensive and instructive 

 Bridgewater Treatise, I find the following differences :— 



The neck, in Hawkins's species, is hardly more than three times the length of the head ; while, 

 in the PL Dolichodeirus it is four times that length. Mr. Conybeare states that the neck in 

 the Duke of Buckingham's specimen is fully equal in length to the body and tail ; but in Haw- 

 kins's Plesiosaur, the length of the neck only, slightly exceeds that of the body or trunk ; and 

 this difference depends both on a difference in the number as well as in the form of the cervical 

 vertebrae. 



The cervical vertebrae in the PL Dolichodeirus, reckoning as such those which supported hatchet- 

 shaped, and not rib-like, lateral appendages, are, according to Mr. Conybeare, thirty- five in num- 

 ber ; while the corresponding vertebrae in Hawkins's Plesiosaur are twenty-nine in number. The 

 cervical vertebras in the latter are also shorter in proportion to their breadth than are those of 

 the PL Dolichodeirus. The head in Hawkins's Plesiosaur is relatively longer than in the PL Do- 

 lichodeirus, equalling a tenth part of the total length of the skeleton ; while in the PL Dolicho- 

 deirus it forms, Mr. Conybeare says, less than a thirteenth part of the same. 



A more readily appreciable difference is presented in the forms and relative sizes of the ulna 

 and tibia in these nearly allied species. In the PL Dolichodeirus, the ulna, or posterior of the two 

 bones which succeed the humerus, is as long as the radius ; and its margin next the radius is but 

 slightly concave. In Hawkins's Plesiosaur the ulna is shorter than the radius, broader in propor- 

 tion to its length, and with a deeper concavity on its inner margin. 



In Hawkins's Plesiosaur, the fibula, in regard to its relative length and breadth, and its bent or 

 reniform figure, and particularly with respect to the curvature of its outer margin, deviates in a 

 greater degree than the ulna from the corresponding bone in the PL Dolichodeirus. 



The differential characters afforded by the bones of the fore-arm and leg are the more satis- 

 factory, because, as we shall presently see, the Plesiosaurus Macrocephalus again presents different 

 and characteristic forms of the same bones. There are other and slighter differences in the shape 

 of the hatchet-bones, or cervical ribs, of the humerus, and of the femur ; but as the aim of the 

 present paper is restricted to the characters of the skeleton of the Plesiosaurus Macrocephalus, 

 my object will be gained if it be admitted that sufficient proof has been adduced of the specific 

 difference between the Plesiosaurus Dolichodeirus and the species with which I wish more imme- 

 diately to compare the Plesiosaurus Macrocephalus. 



The skeleton of the Plesiosaurus Macrocephalus (PI. XLIII.) now under 

 consideration rests upon its under or ventral surface, with the neck and head 

 bent to the left side, so as to describe, with the rest of the vertebral column, 

 almost a semicircle, indicating the extent to which the living animal must have 

 enjoyed the power of lateral inflections of the neck. 



The exposed part of the skeleton gives a view of the upper surface and 

 right side of the head, which is, however, somewhat crushed and mutilated. 

 A similar view is obtained of the cervical vertebras ; but these are in general 

 perfect, and so turned as gradually to expose more of the dorsal surface at 



