EEPORT ON THE TETRACTINELLIDA. 
Ixxxix 
distilled water they were touched for an instant with a piece of blotting paper, introduced 
into a dry weighing bottle, and their weight obtained direct. 
The sponge was next dried in the water oven and again weighed. The specific 
gravity of the dried sponge being taken as two — and this a very close approximation — 
one-half of the weight of the dried sponge was deducted from the first weighing, and the 
remainder represents the weight of a volume of water equal to that of the total volume 
of the sponge. 
To determine the quantity of contained silica, the sponge was next boiled in strong 
nitric acid till all traces of organic matter were destroyed, and the spicules remaining 
were separated by filtration ; after well washing they were transferred to a platinum 
crucible, and the ashes of the ignited filter-paper added ; to eliminate water in 
combination they were then ignited, at first over a Bunsen, and finally with a Herapath. 
The weight of the ignited spicules gave the w'eight of silica present in the sponge. 
Having obtained the weight of silica present in a given sponge of known volume, we 
proceed as follows : — The specific gravity of flint varies from 2*5 to 2 ’6 ; taking the higher 
number, if we multiply by it the number representing the volume of the sponge, we 
shall obtain the weight of a flint of equal size, and can then directly compare the 
quantity of silica furnished by the sponge to that which would be required for the 
formation of a flint equal to it in size ; an example will make this clear. 
Sponge taken — Anihastra pyriformis, Sollas. 
Weight of the sponge full of water, 1’9203 grammes. 
Weight of the sponge when dried, 0*788 gramme. 
1*9203 — (0*788-^2) = 1*5263 grammes, 
which is the weight of a volume of water equal to that of the sponge. This multiplied 
by 2*6, the specific gravity of flint, gives 3*96838 grammes, the weight of a volume of 
flint equal to that of the sponge. The v/eight of the ignited spicules is 0*6658 gramme ; 
and 
0*6658^3*96838 = 1677 per cent. 
From this it appears that the specimen of Anthastra pyriformis examined contained 
16*77 per cent, of the quantity of silica required to convert it into solid flint. Expressed 
as a common fraction this is about -l-th, so that it would require six equal and similar 
sponges to furnish sufiicient silica for the conversion of one of them into flint. 
In making use of the material at my disposal, I selected such sponges as possess 
spicules resembling those found associated with the flints of the Chalk, and in the 
following table the quantity of silica they were found to contain is expressed, as in the 
example just given, as a percentage of that required for the complete conversion of the 
sponge into flint. 
(ZOOL. CHALL. EXP. — PART LXIII. — 1888 .) 
Err m 
