POSTSCRIPT. 
y Whilst this work was going through the Press, I received 
from Professor Sollas, a separate copy of his paper on the Fiznt 
Nodules of the Trimmingham Chalk, which appeared in the No- 
vember and December numbers of the Annals and Magazine of 
Natural History. In this paper, Prof. Sollas has described diffe- 
rent forms of sponge spicules derived from material of the same 
character, in all essential respects, as that from Horstead. So 
far as one can judge from the outlines of the spicules in the 
accompanying plates, they are, with few exceptions, of similar 
forms to those which I have herein described, as also to those from 
the Chalk of Westphalia and the North of Ireland, figured by 
Zittel and Wright respectively. ‘The Trimmingham nodules, how- 
ever, on the supposition that they have been thoroughly search- 
ed, do not contain anything like the variety of spicular forms 
which are present in the Horstead flint. Prof. Sollas has placed 
then under 17 genera, of which no fewer than 13 are mew; 
whereas I have placed the far greater number of spicules from 
Horstead, including nearly all the forms present at Trimmingham, 
under genera already known, to which they appeared closely 
allied. As these spicules so nearly resemble those of sponges 
already described, I am unable to see either the necessity or 
advantage of instituting new genera to contain them, and have 
therefore grouped under one genus, spicules, which Prof. Sollas 
has placed under two or three genera. 
The well-marked manner in which the skeleton of Lithistid 
sponges is built up by the interlocking of the modified arms of 
