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ON BRITISH FOSSIL REPTILES. 195 



teeth. The Ceiiosaurus surpasses all modern Crocodiles in its enormous 

 bulk, which almost rivals that of the Whales, its successors in the modern 

 seas. 1 he genus Streptospondylus, 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 forma- 

 tion 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 freshwater 

 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 combined 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 Jguanodon, Hylceosaurus, Goniopholis, and Suchosaurus, 

 but find that the Megalosaurus, Poikilopleuron, Cetiosaurus, Streptospon- 

 dylus, and Plesiosaurus, 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 modifications and varieties of form and bulk, 

 under which it existed in the oolitic periods ; especially in the older divisions 

 of the oolite, as the lias. The number and variety of Plesiosaurian Reptiles 

 are even more surprising, and the modifications of their skeleton being more 

 marked and various, proportionally 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 subgeneric distinction. This sub- 

 genus, the Pliosaurus, characterizes the Kimmeridge and Oxford clays, but 

 appears not to have existed at the period of the lower oolite. 



In the place of the Goniopholis and Suchosaurus, the Crocodilian genera, 

 Sfeneosaurus and Teleosaurus, with the subgenera, Aelodon, Mystriosaurus, 

 Macrospondylus, &c. (separated, perhaps, without sufficient reason, from the 

 first two typical genera of Amphicoelian Crocodiles), make their appearance 

 in the oolitic strata, especially in the lower divisions. The long and narrow 

 snouts, sharp and slender teeth, short fore-limbs, and imbricated scutation of 

 these extinct Crocodilians, attest, with their vertebral structure, their adapta- 

 tion to an aquatic life, and to the capture of a prey not more highly or- 

 ganized than fishes. 



Some small species of Crocodilians and Lacertians have left a few bones 

 of their extremities in the oolitic slate of Stonesfield ; and a most singular 

 order of Reptiles now makes its appearance, the skeleton of which exhibits 

 a modification of the Lacertian type of structure closely analogous to that 

 by virtue of which the mammalian Bat is endowed with the powers of fiight. 

 Tiie flying Dragons, called Pterodactyli, were of small size, and are restricted, 

 like the Teleosauri and Steneosauri, to the oolitic group. All the other genera 

 are continued into the Wealden, — the Poikilopleuron and Megalosaurus, by 

 identical species, — the other genera by species which are distinct from those 

 of the oolite. The Plesiosaurus and Ichthyosaurus existed, as we have seen, 

 as late as the deposition of the chalk. The analogy between the extinct Rep- 



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