BRITISH SP0NGIADJ1. 171 



Habitat. Three miles off Dunstanborougli, Rev. A. M, 

 Norman. 



Examined. In the dried state. 



At the first sight, this sponge might readily be mistaken 

 for a variety of Polymastia mammillaris. It is about two 

 inches in diameter, and does not exceed half an inch in 

 thickness, and its surface is abundantly furnished with 

 the mammaeform organs, some of which exceed half an 

 inch in length, two lines in diameter, and about one line 

 at the distal extremity. Others are broader, shorter, 

 and more conical, no two appearing to be precisely alike 

 in form and proportions. Their anatomical structure is 

 also entirely different from the similar shaped organs on 

 Polymastia, their interior being filled with the same de- 

 scription of skeleton tissue as that forming the body of 

 the sponge, and this mass of interstitial structure is tra- 

 versed by several excurrent canals, which run nearly parallel 

 to each other from the base of each of the mammae to the 

 apex. The interior structure of the sponge is also re- 

 markable, the abundance of spicula in the membranes of 

 the skeleton, their consequent thickness, and the large size 

 of the intermarginal cavities, causes the specimen in the 

 dried condition to simulate very closely the appearance of 

 the skeleton of a Desmacidon ; but a microscopical ex- 

 amination of its structure speedily relieves us of any 

 doubts regarding its true character. 



This species in the dried state may be readily mistaken for 

 H. consimilis by a hasty examination, but for the difference in 

 its colour, and in the proportions of its spicula as compared 

 Avith those of that species which are in length only as four 

 to seven in the sponge under consideration. The posses- 

 sion of external defensive spicula also, which do not exist 

 in H. consimilis, serve still further to distinguish them. 



No oscula could be detected on the mass of the sponge, 

 but from the structures of the mammae their absence 

 might be expected, the numerous and large longitudinal 

 canals on those organs sufficiently indicating their office 

 and the position of the oscula, although not visible under 



