270 A MONOGRAPIl OF THE 



Habitat. On Pecten opercularis, five fathoms, Belfast 

 Bay, Mr. William Thompson. Isle of Cumbrae, Clyde, Rev. 

 A. M. Norman. Diamond Ground, off Hastings, J. S. 

 Bowerbank. 



Examined. In the living and dried states. 



The first specimen of this sponge was sent to me by my 

 late friend, Mr. William Thompson, of Belfast, who dredged 

 it in April, 1848, in five fathoms water, in Belfast Bay. 

 I have received it also from the Rev. A. M. Norman. In 

 both cases it is exceedingly thinly spread over the sur- 

 face of one or both valves of Pecten opercularis and does 

 not exceed half a line in thickness in any specimen that I 

 have seen. 



Mr. Thompson describes the colour of the sponge as 

 " reddish orange when recent." The Rev. Mr. Norman as 

 " scarlet when living." In four specimens I have examined 

 in the dried state, the colour is ochreous yellow, and all of 

 them appear as if the shells had been washed over with a 

 thick infusion of yellow meal and then dried. 



The oscula are not visible without the aid of a lens of 

 about two inches focus, and many of them are so minute 

 as to be scarcely discernible with that power ; they are 

 numerous, and dispersed over all parts of the sponge, I 

 could not detect any open pores with a power of 160 

 linear. 



The dermal membrane is furnished abundantly with 

 spinulate spicnla which are thickly matted in its inner 

 surface ; they are much shorter and more slender than those 

 of the skeleton, a few of which are intermixed with them 

 and closely imbedded in the same surface; there are nume- 

 rous small inflato-cylindrical spicula. 



I do not recollect having before found the small inflato- 

 cylindrical spicula in any other sponge excepting Chalina 

 feus, on the dermal membrane of which they are exceed- 

 ingly numerous ; they are nearly of the same size in both 

 sponges, but very much less in number in the species under 

 consideration. 



On the external surface of the sponge sent to me by 



