the Chalk, Greensands and Oolites. 183 



sponge remains, its former presence is indicated by the siliceous matter exhibiting 

 the aspect of a congeries of gelatinous globules, evidently arising from the silex having 

 been moulded by the tissue amid which it was deposited ; and the gelatinous mole- 

 cules, when traced to the edges of small patches of the spongeous texture, are found 

 to agree in size and form with the orifices of the supposed in-current canals. In other 

 cases, no indication of the former presence of the organized structure of the sponge 

 remains ; but its previous existence is proved by the mode in which the spicula, the 

 foraminated shells and the other extraneous matters are found suspended, equally 

 in all parts, and not precipitated to one particular portion of the flint, which would 

 have been the case, had the space in which they were deposited been a mere hollow, 

 arising from the imbedment and subsequent rapid decomposition of the body which 

 has given rise to the peculiar form assumed by the mass ; but, on the contrary, 

 their continued suspension in their natural position indicates that the organized 

 tissue in which they were entangled, retained its form and texture sufficiently long 

 to allow of the infiltration and perfect fossilization of these interesting remains, 

 in their original places. Had we no other evidence, the nature and position of 

 these remains would have strongly indicated the former spongeous nature of the 

 tuberous- shaped flints of the cretaceous beds. If the chalk be carefully washed 

 from the exterior of a flint with a soft brush, and a portion of that surface be 

 examined as an opaque object by direct light, with a microscopic power of about 

 50 linear, we shall observe it to present a peculiar saccharine appearance, with 

 occasionally deep circular excavations ; and with small fragments of shells and 

 other extraneous matters, partly imbedded in, or only slightly adhering to, its sur- 

 face. If the surface of the flint be further cleansed from the chalk, by immersion 

 in dilute muriatic acid, until effervescence ceases, the deep circular orifices will 

 frequently be seen to have spicula projecting from their sides ; thus strongly point- 

 ing them out as having been, in all probability, the mouths of the larger ex-current 

 canals. If small pieces, about a quarter of an inch in diameter, be now selected, 

 presenting the roughest and most saccharine aspect, and these be illuminated by 

 a leiberkuhn, and examined with a microscopic power of 120 linear, under favour- 

 able circumstances, it will be seen that the surface is formed by a complex mass 

 of small contorted tubuli, the apices of many of them being furnished with a mi- 

 nute perforation, as represented in fig. 2, PI. XVIII. *= The tubuli on the surface 



River, preserved in spirit immediately after being taken from its place of growth, that siliceous spicula 

 occur in great numbers in the fleshy substance that fills up the horny network of the sponge in its live 

 state. It therefore ceases to be an anomaly, as it might have hitherto been supposed, that siliceous spi- 

 cula should be found imbedded in the fossilized remains of a Keratose sponge. — March 1841. 



* A similar tubular structure of the fibre exists in the recent Spongia Jistularis, described and figured 

 by Dr. Grant in vol. ii. of the New Edinburgh Philosophical Journal. — March 1841. 



