72 Professor Owen ofi BritUk Foasil lieptiks. 



almost rivals that of the whales, its successors in the modern 

 seas. The genus Streptospondylm, which, in repeating the 

 ball and socket structure, offers the strange anomaly of an 

 anterior position of the ball and a posterior one of the socket, 

 makes its first appearance in the Wealden by a species which 

 must have been little inferior to the Cetiosaurus in length. 



The huge terrestrial saurians of the Wealden deviate in so 

 much greater a degree than the crocodilians from the existing 

 types, as to render the formation of a distinct order for their 

 reception necessary. 



It does not appear that any of the Chelonians of the Wealden 

 period are specifically identical with those of the chalk. A new 

 and singular osculant genus, Tretosternon, here represents the 

 Trionyces of the eocene fresh water or estuary formations. A 

 new species of turtle with an emydian form of shell, occurs in 

 the Purbeck limestone ; and the head of a turtle from the 

 Portland stone, upon which the Purbeck beds immediately 

 rest, exhibits the same distinction of the separate nasal bones 

 as does the skull of the turtle from the greensand, but com- 

 bined with well-marked specific differences in other parts. 



The Portland stone introduces us to the great oolitic series, 

 in which we lose sight of the Iguanodon^ Ilylceosaurus, Go- 

 niopholis, and Suchosaurus, but find that the Megalosaiirus^ 

 Poikilopleuron, Cetiosaurus, Strep tospondylus, and Plesiosmi- 

 rw^, are genera common to the Wealden and oolitic periods. 



Now also the genus Ichthyosaurus, which was represented 

 by a single species in the chalk epoch, astonishes us by the 

 number of individuals, and the great variety of specific modi- 

 fications and varieties of form and bulk, under which it exist- 

 ed in the oolitic periods, especially in the older divisions of 

 the oolite, as the lias. The number and variety of plesiosau- 

 rian reptiles are even more surprising, and the modifications 

 of their skeleton being more marked and various, proportion- 

 ally facilitate the determination of the species. The largest 

 of these plesiosaurian reptiles deviates, indeed, so far from 

 the typical structure of the genus, as to merit sub-generic dis- 

 tinction. This sub-genus, the Pliosaurus, characterises the 

 Kimmeridge and Oxford clays, but appears not to have ex- 

 isted at the period of the lower oolite. 



