FOSSIL SHELLS OF THE WEALDEN. 299 



worn bones of reptiles, and pebbles, gravel, and 

 other drift, and are often imbedded in the fluviatile 

 conglomerate which occurs in some parts of the 

 Wealden.* They appear to have floated down 

 the river with the carcases and limbs of the rep- 

 tiles, and at length to have sunk to the bottom 

 and become imbedded in the mud and sand of the 

 delta. 



Fossil shells. — The shells of the Wealden 

 of the Isle of Wight belong to the same genera 

 and species of fluviatile mollusca, as those in the 

 contemporaneous deposits of the south-east of 

 England : and though spread over areas of con- 

 siderable extent, and through deposits of great 

 thickness, they comprise but a few species — a 

 characteristic feature of the fauna of all rivers. 

 The substance of the shells in the limestones is 

 for the most part changed into carbonate of lime ; 

 but the shells in the argillaceous strata are often but 

 little altered, and sometimes occur in as perfect 

 a state as in very recent deposits ; the epider- 

 mis and even the horny ligament being preserved. 

 In the coarse limestones the shells have generally 

 perished, and casts only remain. 



Sussex and Petworth marble. — The most 



* See remarks on the " Discovery of the Hylaeosaurus," in Wonders of 

 Geology, vol. i. p. 401. 



S 



