468 On the supposed Sponge Spicules in Flint. 



somewhat suddenly, having frequently what engravers call a 

 " diamond point " given to them by the introduction of a fresh 

 plane at the apex. This peculiarity of contour had previously 

 attracted my notice in the flint specimens. When the length of 

 each prism has been less than the thickness of the lamina of 

 shell of which it constituted a part, it is pointed at each extre- 

 mity j but when this has not been the case, one end is truncate. 



These facts lead to some interesting conclusions as to the part 

 played by the larger mollusks in the accumulation of calcareous 

 strata. In some recent deposits I have found, that in addition 

 to calcareous Foraminifera, disintegrated shell-structiu'es consti- 

 tuted an important part of the mass, by the decomposition of their 

 animal textures and the separation of their prismatic portions. 

 Dr. Carpenter has shown us in the case of the shell furnished to 

 him by Mr. Stutchbmy (see Report, ut supra, p. 5), that expo- 

 sure to the action of water during a lengthened period is capable 

 of decomposing the animal membrane and causing the calcareous 

 prisms to separate. When we remember the small size of these 

 prisms in many shells, as in some species of Vulsella, where Dr. 

 Carpenter says they are only the 2 ^Vo*^ ^^ ^^ ^^^^ i^ diameter, 

 we can readily conceive how, under favourable circumstances, 

 they may add to the organic elements of limestone rocks, and yet 

 be liable, from their small size and crystalline appearance, to be 

 mistaken for inorganic atoms. 



As this interesting subject is one to which I hope shortly to be 

 able to recur, I will content myself for the present with directing 

 the attention of microscopic geologists to it. Whether or not 

 any real traces of sponge-spiculse are to be found in the chalk, is 

 a question about which I am as yet doubtful. I have not hitherto 

 succeeded in meeting with one which I could without hesitation 

 refer to that class of organisms. I may remark as an additional 

 argument, that whilst I have observed these prisms to be abun- 

 dant in some of the Cambridge chalk, where sponges are compa- 

 ratively rare, I have not as yet seen them in those portions of the 

 Yorkshire chalk, where shells are seldom met with, but where 

 sponges [Alcyonia and Ventriculites) abound. 



I remain, Sir, yours truly, 

 William C. Williamson. 

 78 Bloomsbury, Manchester. 



P.S. Since writing the above. Dr. Mantell has kindly supplied 

 me with an interesting specimen of chalk, taken from the interior 

 of a hollow flint, abounding with Rotalia and Textilaria, and also 

 in the calcareous prisms. Along with these I succeeded in find- 

 ing a very few well-marked calcareous triradiate sponge-spiculse, 

 as well as some small siliceous muricated forms. These however 

 are entirely diff'erent from the prisms so common in ordinary 

 flint. 



