microscopic examination of very many sections diil not reveal the 
presence of any form of spiculum, they were most likely allied to 
the recent keratode sponges, in fact, a thin slice of ordinary 
domestic sponge greatly resembles a section of his silicilied prede- 
cessor. 'I'he reticulations are not solid but tubular, and I have 
been able, in many cases, to fdl them with colouring matter. 
The following figures are from camera lucida drawings of sections 
of Polypothecia, No. 1, from the green sand, \Varmin.ster ; No. 2, 
from a fragment obtained from Mr. Colman’s artesian well, at 
Carrow, Norwich. 
Ihe following extract from a paper on the process of Silicifica- 
tion of Animals, reai.l before the Geological Association last June, 
by Mr. H. M. Johnson, F.G.S., may perhaps be of interest. The 
author points out “ how a crop of sponges invested with their 
gelatinous flesh or sarcode, and liWng at the bottom of a deep 
ocean, were suddenly buried in a thick stratum of white mud con- 
sisting of the minute shells of foraminifera, that they then died, 
and that while in the process of decomposition this interchange of 
materials took place ; the nascent carbonic acid parting with its 
carbon in exchange for the silica of the silicate of soda which sea 
water is known to contain.” 
To illustrate the [lower possessed by decomposing organic matter 
1 . 
o 
Polvpothocia x 150 diameters, 
yreen sand, Warminster, Wilts. 
Polypothecia x 150 diameters, 
green sand, Carrow, Norwich. 
