BRITISH SPONGIADiE. 201 



the difficulty, as there is a striking similarity of form be- 

 tween the spieula of the two species under consideration ; 

 but although this close approximation in form exists there 

 is so great a difference in their proportions as to render 

 their ultimate separation distinct and certain ; so much so 

 that an experienced observer may readily determine the 

 species by the spieula alone. The following measurements 

 derived from the spieula of the type specimens of the two 

 species in the Johnstonian collection in the British Museum, 

 will best illustrate their differential characters. One of the 

 largest spieula from //. carnosa measured ~ s th inch in 

 length, and g^nd inch in diameter. An average-sized 

 one was ^th inch long, and ^th inch greatest diameter, 

 while in H. suberea, one of the longest spieula I could find 

 measured ^th inch in length, and ^rd inch in diameter, 

 and an average -sized one T ~rd inch long, and jnUth inch 

 greatest diameter. I have examined a considerable number 

 of specimens of each species, and I have found their pro- 

 portional differences to exist in every case. In both sponges 

 the spieula are subject to malformation and of irregularities 

 in the development of the spinulate bases, and therefore the 

 differences in their proportions become the more valuable 

 as differential characters. 



A favorite localitv of II suberea is on the shells of 

 various species of Turbo, Fusus, and other univalves, which 

 it often entirely envelopes, intruding itself frequently far 

 into the interior of the shell, although it may be also 

 occupied by a Pagurus. In a specimen assuming the form 

 of a compressed Hymeniacidon feus, and of about or rather 

 larger than the usual size of that species, partially enveloping 

 a shell of a Fusus, the mollusc was evidently alive at the 

 time of the sponge locating itself, as the whole of the face 

 of the shell, from the apex of the whorls to the extreme 

 point of the canal for the passage of the siphuncle of the 

 mollusc, is perfectly clear of the sponge, while the whole of 

 the back of the shell is as completely covered by it ; but 

 notwithstanding that it does not at any part pass over the 

 edge of the shell, it has not succeeded in gaining possession 

 of the interior within the last whorl for nearly an inch, 



