1% Professor Owen on British Fossil BepHles, 



have yet been discovered. This circumstance corresponds 

 with the more strict adaptation for marine existence which 

 the structure of the Ichthyosaurus presents, and corroborates 

 the inference that the Plesiosaurus lived nearer the shore, and 

 ascended estuaries. The reappearance of the Ichthyosaurus 

 in the chalk formations proves that it had continued to exist 

 in the neighbouring ocean, and indicates, perhaps, that the de- 

 position of the cretaceous beds was related to the formation of 

 the Wealden group by proximity of time as well as place. The 

 terrestrial group of gigantic reptiles receives in the Wealden 

 an accession of two new genera, viz., Hylceosaurus and Mego- 

 losaurus ; and the remains of both these, and especially of 

 the lyuanodon, are so abundant, that the Wealden strata may 

 be regarded as the metropolis of the Dinosaurian order.* 

 - The amphibious crocodiles might be expected, from their 

 known habits at the present day, to have left abundant evi- 

 dences of their remains in strata which seem to have been 

 deposited at the estuary or mouth of some great river ; in a 

 climate, indicated by its vegetable fossils to have been warmer 

 or more equable than at present ; and during a period of time 

 which permitted the accumulation of 1000 feet of strata. 

 Accordingly, the crocodilian order of reptiles has been found 

 to be represented by several distinct genera in the Wealden 

 formations. 



Some new characters and modifications of structure might 

 also have been anticipated in those crocodilians which existed 

 at a period antecedent to the deposition of about 1500 feet of 

 cretaceous strata, which again preceded the formation of the 

 wbole series of superimposed tertiary and diluvial beds. 

 Nevertheless, the remarkable modifications which all the 

 Wealden crocodilians present in the structure of their verte- 



• Dr Mantell calculates that not less than seventy individuals of the 

 Tguanodon, varying in age and magnitude, from the young just escaped from 

 the shell to the mature animal, with a femur of more than a yard in length, 

 have come under his examination ; and he justly observes that " more than 

 thrice that number have, in all probability, been destroyed by the workmen, 

 and altogether eluded the observation of the paleontologist." See his Me- 

 moir in the Philosophical Transactions, 1841. 



