POSTSCRIPT. 



Whilst this work was going through the Press, I received 

 from Professor Sollas, a separate copy of his paper on the Flint 

 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 new; 

 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 



