218 UPPER, OR FLINTY CHALK. 



vicinity of Lewes, but the specimens hitherto collected, are not sufficiently 

 perfect to warrant their specification. 



92. Parasitical bodies in the shells of various species of Inoceramus. 

 Tab. xxvii. fig. 7. 



The shells of the larger Inocerami appear to have been subject to the 

 ravages of a peculiar parasitical animal, which destroyed the intermediate 

 substance, leaving the outer and inner plates entire, and supported only 

 by thin partitions. The specimens exhibiting these appearances, are 

 fuU of small oblong cells, connected by linear perforations ; and these are 

 either empty, or filled with chalk, or flint ; in the latter case, they give rise 

 to a curious class of fossils, the nature of which has but very lately been 

 explained. 



A specimen of this kind is represented Tab. xxvii. fig. 7 ; it is part 

 of a flint, moulded in the interior of an Inoceramus, containing on its 

 surface, numerous irregular oblong bodies, more or less compressed, and 

 united to each other by slender lateral filaments. 



These curious bodies were first noticed by Mr. Parkinson * ; and have 

 subsequently formed the subject of an interesting memoir from the pen 

 of the Eev. W. Conybeare, published in the 2d vol. of the Geological 

 Transactions. 



The investigations of Mr. Conybeare, have clearly elucidated the origin 

 of the fossils in question, and shewn " that they are siliceous casts, formed 

 in little cells, excavated in the substance of certain marine shells, the work 

 of animalculge preying on those shells, and on the vermes inhabiting them. 

 These casts, hke the screw-stones of Derbyshire, must have been formed by 

 the infiltration of siliceous matter while in a fluid state into the cavities of the 

 shells, and which have been laid open and denuded by subsequent exposure 

 to some agent, capable of dissolving and removing the calcareous matter of 

 the shell forming the matrix, while the siliceous cast remained unaltered f ." 



* Organic Remains, Vol. ii. pp. 75, 76. 

 •f Geological Transactions, Vol. ii. p. 328. 



In a very interesting fossil in my possession (from St. Peter's mountain, near 

 Maestricht,) changes of a similar nature have taken place, but with this difference, that the 



