182 Mr. BowERBANK on the Siliceous Bodies of 



careful investigation, in any chalk flint, without reference to the spot from which 

 my specimens may have been procured. 



When thin polished slices of the common tuberous flint of the chalk are pre- 

 pared by a lapidary, mounted upon slips of glass, and subjected to examination, 

 as transparent objects, with a microscopic power of about 120 linear, it will be 

 observed, that the appearance presented to the eye is similar to that of a thin stra- 

 tum of a turbid solution of decomposed vegetable or animal matter, containing frag- 

 ments of extraneous bodies, mixed with small foraminated shells, spicula of sponges, 

 and minute animalcules ; especially of that highly interesting and beautiful genus, 

 Xanthidium. Amid this heterogeneous mass there are frequently to be noticed 

 fragments of the brown reticulated tissue, to which I have before alluded ; and 

 occasionally, if the slice be from near the surface of the flint, these patches of 

 brown spongeous substance are comparatively of considerable size, and are dense 

 and opaque. In this state, and by this mode of examination, the reticulated form 

 of the structure can only be perceived at the edges of the mass. When it occurs 

 in this well-preserved form, which is indicated to the unassisted eye by the appear- 

 ance of rusty brown coloured spots on the flints, it is best examined by placing a 

 small round black patch, about two lines in diameter, behind the spot to be exa- 

 mined ; and then illuminated by a leiberkuhn ; and viewed with a power of 120 

 linear, it exhibits the characters represented in fig. 1, PI. XVIII. In that speci- 

 men, which is from a flint procured at Northfleet, near Gravesend, the struc- 

 ture is remarkably well preserved. The mass of the sponge appears to con- 

 sist of numerous cylindrical contorted canals, which, from the uniformity of their 

 size and the minuteness of their diameters, would appear to have been the in-cur- 

 rent canals of the sponge. Occasionally orifices of a considerably larger diameter 

 are dispersed in the mass, and have every indication of having been the large 

 ex-current canals. The walls of these canals present the appearance of having 

 been formed of a thin glairy network, bearing a strong resemblance to the reticular 

 substance of the common freshwater sponge of the rivers of England. Amid the 

 mass are found spicula, comparatively speaking, very sparingly dispersed ; and 

 minute foraminated shells of many species, imbedded in a precisely similar man- 

 ner to what we observe on a larger scale in the recent Mediterranean sponge of 

 commerce*. Very frequently, when but little of the reticulated substance of the 



* When the above passage was written, it was generally considered that the Keratose sponges were 

 destitute of spicula ; but I have since ascertained that they are occasionally found imbedded in the larger 

 horny fibres of the sponges of commerce, and in numerous other sponges of the same genus from New 

 South Wales ; and I have described the mode in which they occur in a paper read before the Micro- 

 scopical Society, Jan. 27, 1841. Since the publication of these facts, I have also ascertained, from the 

 examination of an Australian species of Keratose sponge which was brought by Mr. Gould from Swan 



