774 DR. J. S. BOWERBAXK ON THE SPONGIAD.E. [Nov. 21, 



without contact with any other portions of its structures, the spicula 

 of the termination radiate on the plane of the membrane like those 

 which are projected through the dermis. This habit of terminal 

 radiation of the fasciculi prevails in several other species of Raphio- 

 desma. The tension-spicula are very unevenly distributed ; in some 

 parts there are very few of them, while in others they occur dispersed 

 in very considerable numbers. 



The rosette-shaped groups of dentato-palmated inequianchorate 

 retentive spicula are rather sparingly but equably dispersed on the 

 inner surface of the areas of the dermal network. They very closely 

 resemble those of the type of the genus, the British species Rap/iio- 

 desma lingua, both in the size of the groups and of the individual 

 spicula of which they are constructed, but they are not nearly so 

 numerous in each group as in the type specimen. 



I have never found these rosette-like groups of inequianchorate 

 spicula in any other sponges than those of the genus Raphiodesma, 

 and not in every species of that genus ; so that, although they 

 cannot be considered of generic value, they are important indications 

 of the genus whenever they are apparent. A few simple or contort 

 bihamate retentive spicula were occasionally observed on the inter- 

 stitial membranes of the skeleton ; but their number was so small as 

 to render their presence of but little value as specific characters. 



The structures of the skeleton-fasciculi are longer and more con- 

 tinuous than in any other species of the genus with which I am 

 familiar, so much so that the skeleton might very readily be 

 mistaken by a hasty observation for that of a Deswacidoii ; but this 

 illusion is dissipated by the fact that they do not form a continuous 

 network of solid fibre as in the last-named genus, but frequently 

 terminate in radial expansions like those of the dermal membrane 

 and its external protected groups of defensive radiating spicula. A 

 few rosette-shaped groups of inequianchorate spicula and a few of 

 the bihamate ones are occasionally found on the interstitial mem- 

 branes. 



This sponge is the only specimen of the species that I have seen ; 

 and it is a very remarkable one. By far the greater proportion of 

 the known species of the genus are more or less of a solid massive 

 form ; and this variation strongly illustrates the fact that there is 

 little or no dependence to be placed on external form in regard to 

 either generic or specific characters. 



EXPLANATION OF THE PLATES. 



Plate LXXTIIT. 



Dcsmacidon plumosa. 



Fig. 1 represents the sponge in its dried state, rather less than half its natural 

 size. 

 2, 3. Two small portions of the sponge, of the natural size, exhibiting the 

 plumous nature of the hispidation of the surface. 



4. Two of the subfusiform acerate spicula of the skeleton, x -"'OS linear. 



5. A small portion of the skeleton-structure. X 50 linear. 



