BRITISH FOSSIL REPTILES. 61 



given by Mr. Conybeare in the second part of the first volume 

 of the second series of the Geological Transactions leaves little 

 to be said excepting in regard to those points in vt'hich it differs 

 from the PL Hawkinsii. 



The head is relatively smaller in proportion to the body; 

 forming less than the thirteenth part of the whole length of the 

 skeleton, while in the PL Hawkinsii it forms less than one 

 tenth part. 



This diminutive head was supported on a longer neck. In 

 the PL Hawkinsii the head is three times the length of the 

 neck ; while in the PL dolicliodeirus it is four times that 

 length. Mr. Conybeare states that the nock of the PL dolicho- 

 deirus is fully equal in length to the body and tail united ; but 

 in Hawkins'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 dolicliodeirus, reckoning as 

 such those which supported hatchet-shaped, and not rib-like, 

 lateral appendages, are, according to Mr. Conybeare, thirty- 

 five in number ; while the corresponding vertebrae in Hawkins's 

 Plesiosaur are twenty-nine in number. The cervical vertebrae 

 in the latter are also shorter in proportion to their breadth than 

 are those of the PI. dolichodeirns. 



The dentary bone has a shorter and less expanded symphy- 

 sial portion, and the anterior teeth have a smaller proportional 

 size than in the PL Hawki?isii or PI. macrocephalns. 



A lower jaw of this species in the collection Miss Philpotts, 

 measures 



In. Lin. 



in length 5 8 



in breadth behind the teeth .... 3 G 

 The number of teeth in this lower jaw was 50 (25-25). 



The spinous processes of the vertebrae are more compressed 

 laterally in the PL dolichodeirns than in any other species of 

 Plesiosaurus which I have seen. 



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 dolicliodeirus, 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 proportion to its length, and with a deeper concavity 

 on its inner margin. 



In Hawkins's Plesiosaur, the fibula, in regard to its relative 



