BRITISH SP0NGIAD.E. 225 



With the assistance of my friend, Mr. Bean, I found this 

 sponge on the underside of small slabs of yellow oolitic 

 sandstone between high- and low-water marks about two 

 miles north of Scarborough; when fresh from the sea, it 

 had the appearance of a thin layer of dark-amber coloured 

 gelatine, and no oscula were visible. In the fresh state, it 

 was scarcely a line in thickness, and, in its dried condition, 

 it is not thicker than a sheet of writing-paper. Professor 

 Dickie's specimen from S trail gford Lough, on a shell of 

 Pecten opercularis, is rather thicker, and is darker coloured 

 than the Scarborough one. 



This sponge was designated Halisarca, by Dujardin, from 

 his believing it to be destitute of spicula ; and my friend, 

 Dr. Johnston, failed to detect them in the specimens he 

 found in Berwick Bay. I am not astonished that they 

 eluded the observation of these eminent naturalists, as the 

 spicula are very slender, and so completely embedded in 

 the sarcode as to render them very difficult of detection 

 when the sponge is in its natural condition, and it is only 

 when a portion of it is immersed in Canada balsam, and 

 with a power of about three hundred linear, that they are 

 rendered distinctly visible. 



The principal portion of the sponge consists of the der- 

 mal membrane, which is covered internally with irregularly 

 dispersed skeleton spicula, amidst which solitary specimens 

 of the defensive spicula may be observed lying in various 

 directions. The interstitial membranes are few, very short, 

 and crowded with spicula. The forms of the spicula are 

 very characteristic of the species, and the defensive ones 

 especially so. 



The entire structure of the sponge is in perfect accord- 

 ance with the other known species of thin coating sponges 

 of my genus Hi/meniacidon, amongst which I have there- 

 fore thought it advisable to place it. The genus Halisarca 

 was founded in a misapprehension of the structure of the 

 animal. Had Dujardin been aware of the existence in its 

 structure of membranes and spicula, such as are now shown 

 to be abundant, he certainly would not have made a sepa- 

 rate genus of it, but woidd have placed it among its 



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