BRITISH SPONGIAD^. 253 



Interstitial membranes. Tension spicula sub-fusiformi- 

 acerate, same as those of dermal membrane, dispersed. 

 Retentive spicula bidentate angulated equi-anchorate, 

 abundantly dispersed. 



Colour. Alive, ochreous-yellow, dry, brown. 

 Habitat. Diamond Ground off Hastings, J. S. Bow- 

 erbank. 



Examined. In the living state. 



This specimen is two and a half inches long, one and a 

 quarter broad, and about three quarters of an inch thick. 

 I received it among several specimens of Halichondria 

 incrustans of similar size and form, and from which sponges 

 I could not distinguish it excepting by a microscopical 

 examination of its structure. The construction of the 

 skeleton is very loose and irregular, and from the abundance 

 of the large acerate tension spicula the sponge may, by a 

 hasty examination, be easily mistaken for a Hymeniacidon ; 

 but its true structure is readily demonstrable when mounted 

 in Canada balsam, by the aid of a power of about 300 

 linear. The skeleton spicula, and the internal defensive 

 ones, very closely resemble each other, but the latter are 

 very little more than half the length of the former, and 

 although smaller, the spination stronger and more abundant 

 than those of the skeleton, and their spination is entire from 

 base to apex, while the apical third of those of the skeleton 

 are usually entirely free of spines. The retentive spicula are 

 minute, but proportionally stout, and are very abundant 

 on all parts of the membranous tissues of the sponge. 



17. Halichondria Dickiei, Boioerbank. 



Sponge. Coating (?) ; surface smooth. Oscula simple, 

 dispersed, small. Pores inconspicuons. Dermal 

 membrane pellucid, abundantly spiculous ; spicula 

 mucronato-cylindrical, occasionally sub-clavate-hastate 

 fasciculated. Skeleton spicula acuate entirely spined. 



