468 On the supposed Sponge Spicule 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 poimted at each extre- 
mity ; 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-structures 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. Stutchbury (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 caleareous 
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 5,',5th of an inch in 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-spiculz 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 (Aleyonia and Ventriculites) abound. 
I remain, Sir, yours truly, 
Wiiram 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 Rotala ‘and Textilaria, and also 
in the calcareous prisms. ‘Along with these I succeeded in find- 
ing a very few well-marked calcareous triradiate sponge-spicule, 
as well as some small siliceous muricated forms. These however 
are entirely different from the prisms. so common in ordinary 
flint. 
