xiii. PLEIOSA UR US. 341 



PLEIOSAURUS BRACHYDEIRUS. Owen. 



The generic title of c Pleiosaurus' is employed by Professor Owen, 

 to whom we are indebted for nearly all that is known of it, to 

 designate marine reptiles with a large head and short neck, and 

 paddles much like those of plesiosaurus. The cervical vertebrae 

 are short and broad, and have (a) simple or (b) divided marks of 

 adherence of the pleurapophyses, these being robust and more or 

 less cylindrical in figure. All the vertebrae are either plane or 

 slightly concave on their articulating surfaces. The caudal ver- 

 tebrae, as far as they are known with certainty, are short and broad. 

 In several respects, as will be seen, this ' generic' group looks 

 toward ichthyosaurus, which it rivals and even surpasses in 

 magnitude. 



The most important of all the specimens of large pleiosaurians 

 yet discovered is certainly that which was found at Market-Easen, 

 in Kimmeridge clay, as usually admitted a . It furnishes indeed 

 almost the only authentic collocation of the head with other parts 

 of the body, and thus becomes a kind of index to the separate parts 

 in other less complete examples. It contains the head ; both jaws ; 

 eight or ten vertebrae of the neck ; ten of the back ; one of the 

 tail. There are large bones regarded as femora by Owen, and 

 several paddle bones. Some other parts of the skeleton are pre- 

 served in stony connection with a series of dorsal vertebrae b . 



The head, much shorter than the lower jaw, owing to the far 

 extension backward of the branches of the latter, is somewhat 

 crushed, but shews plainly the upper and inner surfaces, the sockets 

 of teeth to the extent of twenty-six on one side, and several teeth 

 in the sockets. The series of teeth is not complete. 



The teeth were large in the anterior part, with an interspace 

 between the fourth and fifth, and a contraction there of the jaw : 

 after this the sockets for the teeth increase in size to the twelfth, 



a I have had some doubt on this matter, on account of the recurrence of a variety 

 of Ammonites vertebralis, at Market-Rasen ; the same variety being frequent in the 

 upper part of the Oxford clay at Cowley. 



b See Owen, Report on Fossil Reptiles, Part ii. 1841, p. 60. 



