PLESIOSAURIDiE. 191 



skeleton of Plesiosaurus manseli ; this specimen might, indeed, be 

 referable to C. brachistospondylus, but since the latter appears to be 

 a comparatively rare form it is more probable that the commoner 

 type of vertebrae were associated with this humerus. The vertebrae 

 described under the names of Plesiosaurus manseli and P. validus 

 are characterized by a more or less distinct lateral depression on the 

 centra of the cervical vertebrae, while in those described as P. bra- 

 chys^ondylus and P. megadirus such depressions are absent. P. 

 validus appears, however, to be somewhat intermediate in this re- 

 spect between P. manseli and P. brachysjiondylus ; and since similar 

 variations occur in regard to the depth and form of the cupping of 

 the terminal faces of the cervical vertebrae it seems impossible to 

 separate these two types of structure without making at least three 

 undefinable species. The variations in this respect appear, indeed, 

 to be local racial characters, the northern race having lost the 

 lateral depression. Thus in the one known skeleton from Dorset- 

 shire these depressions are very marked (P. manseli) ; in the inter- 

 mediate Oxfordshire area we meet with one form (P. validus) in 

 which these depressions are less marked, and another (P. brachy- 

 sjiondylus) in which they are absent ; while in the Cambridgeshire 

 type (P. megadirus) these depressions are invariably wanting. 



The difference in the size of the type femur and that of the skele- 

 ton of P. manseli can scarcely be taken as a specific character, since 

 it is evident that in species of this size a considerable limit must 

 be allowed for sexual and individual variations in this respect. The 

 genus Colymbosaurus was founded upon the evidence of a detached 

 pectoral girdle from Ely (fig. 59), which was identified by its 

 describer with P. megadirus ; P. manseli being referred by the same 

 writer to a subgenus of Muraznosaurus. In the ' Hep. Brit. Assoc' 

 for 1841, p. 61, Plesiosaurus trochanterius was transferred by its 

 founder to Pliosaurus ; the skull subsequently described under that 

 name is catalogued in Pt. i. p. 104 under the Crocodilian genus 

 Machimosaurus. 



Hah. Europe (England and France). 



42496. An associated series of 80 vertebrae, mostly wanting the 

 neural arches ; from the Kimeridge Clay (Upper Jurassic) 

 of Kimeridge 13ay, Dorsetshire. These specimens are 

 described by Hulke in the ' Quart. Journ. Geol. Soc/ 

 vol. xxvi. pp. 013-015, and together with the under- 

 mentioned coracoids and limb-bones are the types of Ples- 

 iosaurus manseli. Twenty-eight true cervicals now re- 

 main, but several arc missing behind tho axis, and there 



