BRITISH SPONGIADtE. 129 



Henry Ridley, of Hastings, who had them from one of the 

 trawlers. The sponges, each completely covered the upper 

 valve of a Pecten opercularis, and in thickness and general 

 external characters closely resembled each other. The shells 

 were uniformly coated with the sponge, which did not, in any 

 part, exceed the eighth of an inch in thickness. The general 

 anatomical structure, when examined microscopically, so 

 closely resembles that of Microciona carnosa when in a 

 young state, that it might readily be mistaken for that 

 species. The slender, flexuous, and branching characters 

 in the skeleton columns of each are alike, and it is only 

 when we apply a microscopical power to the tissues when 

 mounted in Canada balsam, that the difference in the cha- 

 racters of the spicula of the skeleton and the tension 

 spicula, and the total absence of retentive spicula, which 

 are so abundant in M. carnosa, becomes distinctly appa- 

 rent. 



The basal portions of the skeleton spicula are very 

 sparingly spinous, and the spines are minute ; they rarely 

 extend beyond about two diameters of the spiculum above 

 its hemispherical base. The tension spicula are very long 

 and slender, and are frequently flexuous. The internal 

 defensive spicula are very numerous and vary much in size ; 

 they are very abundant on the columns of the skeleton, 

 especially towards their basal portions. 



4. Microciona armata, Bowerbank. 



Sponge. Coating thin ; surface minutely hispid. Oscula 

 simple, minute. Pores inconspicuous. Dermal mem- 

 brane spiculous, thin, pellucid. Tension spicula sub- 

 clavate attenuato-acuate, long and slender, and angu- 

 loid tricurvato-acerate ; retentive spicula dentato-pal- 

 mate, equi-anchorate and bidentate, equi-anchorate 

 very minute. Skeleton. Columns short and stout; 

 spicula attenuato-acuate, basally spined, large and long. 

 Internal defensive spicula attenuato-acuate, entirely 

 spined, short and stout ; spines of the base numerous, 



9 



