PLATE LXXXV. 293 



The sponge which is the type of this species is a 

 mere fragment, about an inch in length, by a little 

 more than half an inch in breadth, and it presents no 

 external characters by which it could have been recog- 

 nised as a species ; but its internal structural characters 

 are distinctly different from those of the only other 

 British species of Desmacidon, D. constrictus, with 

 which it is possible to confound it. The oscula and 

 pores are not available, but there is fortunately a small 

 fragment or two of the dermal membrane in a good 

 state of preservation. It is pellucid, and rather abun- 

 dantly supplied with tension spicula, and occasionally 

 with a few of the stouter acuate spicula intermixed 

 with them. The tension spicula of the dermal and 

 interstitial membranes are slightly less than those of 

 the skeleton fibre, but when separated by boiling in 

 nitric acid they are not to be distinguished from each 

 other, but the difference between them and the larger 

 auxiliary acuate spicula is very distinct. An average- 

 size skeleton spiculum measured was g^-g- inch in length, 

 and TTFTT inch greatest diameter, while one of the 

 large auxiliary spicula measured -g-yr inch in length by 

 10 Q 00 inch diameter. In contrast with these measure- 

 ments, a skeleton spiculum of D. constridus measured 

 -7-9 inch in length, and 33*33 inch diameter, a difference 

 so great as to at once render apparent the specific 

 difference of the two species; and in addition to the 

 discrepancy in size, there is not the slightest indication 

 of the constriction at the basal portion of the skeleton 

 spiculum of D. incognitus, that is so distinctive a 

 character in D. constrictus. The intermixture of the 

 large, acuate, auxiliary spicula with those of the 

 skeleton fibre is of rare occurrence, but they may 

 be frequently observed cemented to the external sur- 

 face of the fibre. The admixture of these larger 

 spicula in the skeleton fibre and in the membranous 

 structures is an unusual event. Generally speaking the 

 skeleton spicula would be the largest and strongest in 



