BllITISH SPONGIADiE. 303 



with two others, which proved, on examination, to be 

 Isodictya jistulosa ; but the two species so exactly resembled 

 each other in their external characters, as to render it quite 

 impossible, in the dried state, to have distinguished the 

 one from the other. When anatomically examined this 

 difficulty at once disappeared. The greater length of the 

 spicula as compared with those of I. jistulosa, being in the 

 proportion of three or three and a half in the former to two 

 in the latter, at once serves to mark the difference existing 

 between them and the presence of the tricurvate spicula in 

 the membranes, and the absence of dermal armature con- 

 firms this conclusion. The fistular projections on the 

 surface of the sponge are very like those of I. jistulosa, as 

 alluded to in the description of the smaller specimen of 

 that species which I have described, and it agrees closely 

 with it also in size, being an inch and three quarters in 

 length, and an inch in average diameter. The careful ob- 

 servation of these differential characters is very necessary 

 when the external character of two species so closely 

 resemble each other. 



The very diffuse and irregular structure of the skeleton 

 causes it to closely simulate the generic characters of a 

 Halichondria, and it is only near the surface that its 

 structure can be satisfactorily determined, and the com- 

 paratively large size of its spicula strongly favours the 

 illusion generated by the irregularity of its skeleton. The 

 tricurvate acerate spicula are very slender and delicate, and 

 require a microscopic power of two or three hundred linear 

 to exhibit them distinctly. In all of them the middle 

 curve is well developed, but the terminal curves in many of 

 them are very shghtly produced. In some of the areas of 

 the dermal network they are comparatively numerous, while 

 the acerate slender tension spicula are of rare occurrence. 

 The pores are visible by the aid of a lens of two inches 

 focus. 



