256 Mr. J. S. Bowerbank on the Siliceous Bodies of the Chalk. 



in sponge. He can have seen but very few species of tbe recent 

 Spongiadse to be thus surprised ; they abound in such extraneous 

 matters, and moreover are the natural habitation of many species 

 of Balanus and other genera of shells, just as we find among the 

 Corallidse certain genera and species of shells which are familiar 

 to every conchologist as occurring in such situations and in none 

 other. Is it then a wonder that living things should be enve- 

 loped in sponges, either ancient or modern, seeing that in many 

 cases it is their natural and inevitable situation ? 



The author then alludes to the fact of the pulp-cavities of the 

 teeth in the fragment of a jaw of Mososaurus in the possession of 

 Mr. Charlesworth being filled with silex, and quotes this as ini- 

 mical to my views of the origin of flint ; and in this opinion some 

 time since I know my friend Mr. Charlesworth shared ; but after 

 having at the British Association expressed his views regarding 

 this interesting specimen, he with his usual liberality gave me 

 permission to take a thin longitudinal slice from the centre of 

 one of these flint casts of the pulp-cavity, and upon examining 

 this through the microscope in the usual manner it was found 

 to exhibit all the characteristic appearances of flint nodules. Two 

 specimens of Xanthidium and numerous polythalamous shells 

 were imbedded in the midst of it, and a considerable quantity of 

 the remains of sponge tissue is apparent round the edges of the 

 slice. Now it must be borne in mind that the fossil was but the 

 fragment of a jaw when imbedded in the chalk, and I believe, from 

 my recollection of the specimen, that at the time of its imbedment 

 it had lost the lower edge or keel of the bone : is it therefore to 

 surprise us, that such bodies as sponge gemmules, often much less 

 than one hundredth of an inch in diameter, having ciliary loco- 

 motive power, should insinuate themselves into the pulp -cavities, 

 either through ^the nerve or blood channels, or by means of the 

 space between the tooth and its socket after the animal matter 

 lining the latter has been removed by maceration, and there de- 

 velope themselves and fill up the space of the cavity ? Neither is 

 it unnatural, that such minute living bodies as microscopic fora- 

 minifera and Xanthidia should be found in such a situation, as 

 sponges are continually inhaling currents of water through one 

 set of canals and ejecting it as continually through others, and this 

 with no small degree of power. 



When I was at Tenby some years since, I placed some speci- 

 mens of Halichondria panicea, Johnston, in a shallow dish of 

 sea-water, and in one of them in which the orifice of the excur- 

 rent canal was more than half an inch below the surface of the 

 water, the outpouring current was so strong, that when the reflec- 

 tion of one of the bars of the window was brought over the ori- 

 fice of the sponge, the reflected line was curved to a very consi- 



