Geological Society. 225 



ving fragments of niiimte corals aiul small shells attached to i\\v in- 

 ner surface. A thin slice exhibited the usual organic contents of the 

 conunon Hint. He, therefore, infers that the tabular Hint which in- 

 closes the zoophytes, owed its origin also to a sponge which invested 

 the organic nucleus. 



A comparison of the characters presented by the spongeous re- 

 mains of the flint, with a collection of recent sponges, has induced 

 Mr. Bowerbank to conclude that the fossils cannot be referred to any 

 of the established divisions of existing sponges. 



On examining the cherts of the greensand of Fovant in Wilt- 

 shire in the same manner, he found that the only differences between 

 them and chalk flints, existed in the coarser texture of the spongeous 

 fibre, the greater size of the interstices of the network, and the larger 

 dimensions of the imbedded extraneous bodies. The cherty nodules 

 of the upper greensand of Shaftesbury afforded similar appearances. 

 A black, semi-transparent nodule, with an outer coat resembling ag- 

 glutinated sand, Mas found under the microscope to contain nume- 

 rous contorted canals of various sizes, and a considerable number of 

 beautiful green spicula. Two chert casts of Spatangi from Shaftes- 

 bury afforded results analogous to those obtained from chalk Echi- 

 nites. 



Slices from a great variety of the greensand cherts of Lyme Regis 

 presented characters which agreed with the cherts of Fovant. A 

 specimen of flint from the Portland stone of Tisbury, and another 

 from Portland, gave a greater quantity of cellular structure than any 

 of the previously noticed cases, and the texture bore a greater aflS- 

 nity to that of the freshwater sponge, than is exhibited in the flints 

 of the chalk or the cherts of the green sand. 



With respect to the causes of the deposition of the flint, Mr.Bower- 

 bank objects to the supposition, that it was influenced by the silice- 

 ous spicula of the sponges, because the flint is in no case limited or 

 determined by their immediate presence, but is, in all instances, bound- 

 ed by the extent of the animal matter of the sponge. He has fre- 

 quently observed that the large excurrent canals in the chalk-flint 

 spongites are not filled with silex, and that the spicula projecting into 

 them have not the slightest incrustation of siliceous matter upon 

 their surface ; while on the contrary, wherever a single tube or 

 a thin layer of tubes has been projected from the mass into the 

 chalk, the silex has been attracted to it. He conceives also, that the 

 retention of the spicula and extraneous matters in all parts of tlie 

 flint, may be accounted for, by supposing that the animal matter was 

 the attractive agent, acting equally throughout the whole body of 

 the sponge. In support of his argument he adduces the siliceous 

 shells of Blackdown, and the siliceous corals of the Tisbury oolite 

 and the mountain limestone, which contain no spicula, and in which 

 it cannot be supposed that previously existing siliceous matter was 

 the attractive agent. Lastly, the pyi"itous fossils of the London, 

 Kimmeridge, Oxford and other clays, are also mentioned as exam- 

 ples of animal and vegetable substances having exercised an attract- 

 ive influence. 



Ann. ^ Mag. N. Hist. Vol.Vu. Ql 



