124 BRITISH SPONGIAD^. 



" The colour and external appearance of this sponge, 

 and the similarity of its location among the branches 

 of Nullipora polymorplia, would readily lead a hasty 

 observer to the conclusion that the type-specimen of 

 Halichondria flabellifera and this one in course of 

 description were the same species ; but a very slight 

 microscopical examination quickly dissipates the 

 illusion. But there is yet another difficulty to be 

 surmounted before we arrive at a correct determination 

 of the genus and species. The structural peculiarities 

 of this sponge are so wide and diffuse, and the abund- 

 ance throughout the whole of its structure of the long 

 and slender tension spicula, as to very readily lead to 

 the belief that the skeleton structure is that of a 

 Hymeniacidon, and it is only by the careful examination 

 of a correctly cut section at right angles to the dermal 

 surface, and with a power of about 200 linear, that we 

 become satisfied of its isodictyal structure. The 

 specimens under consideration are immersed among 

 the branches of Nullipora polymorplia, the largest 

 being one and a quarter inch in length by three 

 quarters of an inch in diameter, and in some parts it 

 completely covers the distal terminations of the 

 branches of the Nullipora. 



" The dermal characters in this species are remark- 

 able, there not only being a well-developed rete, but in 

 addition to that in many parts an abundant supply of 

 long, slender, acerate tension spicula ; so numerous in 

 some cases as to nearly obscure the reticulate structure 

 beneath them. 



" The dermal rete is very distinctly produced, but its 

 mode of arrangement is very irregular, and the areas 

 assume a great variety of forms, which are frequently 



