BUniSH FOSSIL UEI'TILES. 69 



it is long ; which proportions distinguish it from the fibula of 

 either the PL Haivkinsii or PL dolichodeirus. 



The tarsus consists, in PL macrocephaliis, of six, instead of 

 five bones as in the PL Haivkinsii. It participates in the pe- 

 culiarity of having those bones, which are situated at the ante- 

 rior or tibial side of the joint, much smaller than those of the 

 fibular side, and so placed between the tibia and tibial metatar- 

 sals as to indicate that the foot had a freer inflection forwards, 

 or upon the tibia, than in the opposite direction. 



In the PL Hawkinsii the interspace between the tibia and 

 metatarsals is occupied by a single round flat bone 5 but in the 

 PL macrocephalus by two ; the additional bone being situated 

 at this part of the tarsus. 



The metatarsals resemble in number and disposition those of 

 the PL Haivkinsii. In the general form and proportions of the 

 phalanges of both extremities a close resemblance exists be- 

 tween the two species. 



Localities. — This species occurs in the lias of the valley of 

 Lyme : also, but more rarely, in the lias of Street. I have 

 seen detached vertebrae of the PL macrocephalus from the lias 

 of Weston near Bath. 



The vertebrae of the Plesiosaurus, included by Professor Jae- 

 ger in his list of the fossils of the lias of Boll in Wirtemberg, 

 approach more nearly to the characters of the PL macrocephalus 

 than they do to any other well-determined species. 



Plesiosaiirus brachycephalus. 



This species, in the strength and comparative shortness of its 

 neck, and in the proportions of its extremities, is most nearly 

 allied to the Plesiosaurus macrocephalus ; but it differs from 

 that species in the form of its head, and in the character of its 

 cervical vertebrae. 



In the nearly complete skeleton of the PL hrachycephalus, 

 preserved in the Museum of the Philosophical Society of Bris- 

 tol, 75 vertebrae may be counted, and only a few seem to be 

 wanting from the extremity of the tail. Of these at least twenty- 

 eight may be reckoned as cervical, according to the characters 

 assigned to this series of vertebrae in the introductory part of 

 the present Report. The length of the bodies of these vertebrae 

 does not quite equal their transverse diameter. The vertical dia- 

 meter of the body of the 13th cervical vertebra was 1 in. 5 lines, 

 the antero-posterior diameter 1 in. 2 lines. The anterior and 

 posterior articular surfaces of the body are gently but regularly 

 concave without any median convexity. The costal depres- 



