and Clavicular Arch in Sauropterygia. 151 



podial bones are quadrate or transversely elongated, and the carpals 

 and tarsals are transversely oblong. There is a certain parallelism 

 between this group and the Plesiosauridee, as though it had been 

 modified from it, and the genus Polycotylus in the shortness of its 

 centrum has some resemblance to Rhomaleosaurus, just as in length of 

 neck and vertebras Murcenosanrus resembles Plesiosaurus. 



This scheme of classification is summarised in the following 

 table : — 



SAUROPTERYGIA. 



Aquatic Sauromorpha, with the extremities modified for swimming 

 only. They are divided into two groups : — 



Dickanopleura comprise all genera with fork-headed cervical 

 ribs. They are divided into two groups : — 

 Dolichodeira (or Plesiosauridas) comprise the long-necked 

 genera — - 



Plesiosaurus, in which the carpus is weakly ossified. 

 Eretmosaurus, in which the carpus is strongly ossified 



and the scapnla appears to have an inner and an 



outer union with the coracoid. 

 Brachydeira (or Pliosanridas), the genera with sbort necks — 

 Rhomaleosaurus, in which the cervical vertebras are 



shorter than the dorsals. Articular face concave. 

 Pliosaurus, in which the cervical vertebras are shorter, 



and flat on the articular face. 

 Cercidopleura comprise all genera with single-headed, or skewer- 

 like, cervical ribs. They are not yet divided into 

 groups, and are all included in the Elasniosauridas — 



Polyptychodon. 



Polycotylus. 



Cimoliosaurus . 



Stereosaurus. 



Mauisaurus. 



JElasmosaurus. 



Trinacro mer um . 



Colymbosaurus. 



Murcenosaurus. 



Cryptoclidus. 



VOL. LT. 



M 



