314 PLATE LXXXIX, 



ISODTCTYA INCERTA, Bowerbank. 

 Plate LXXXIX. 



Sponge sessile, more or less fan-shaped. Surface 

 smooth and even. Oscula simple, minute, dispersed. 

 Pores inconspicuous. Dermal membrane abundantly 

 spiculous ; spicula irregularly reticulated ; rete uni- 

 spiculate, occasionally fasciculated, same form and size 

 as those of the skeleton. Skeleton very diffuse and 

 irregular ; primary lines multispiculous ; secondary 

 lines mostly unispiculous, numerous and very irregular ; 

 spicula acerate, rather large. 



Colour. In the dried state, milk white. 



Habitat. Isle of Mull, Rev. A. M. Norman. 



Examined. In the dried state. 



I received four of these little sponges for exami- 

 nation from Mr. Norman ; the whole of them were of a 

 compressed or fan-shaped form, and of thickness of 

 about two or three lines, the figured one being the 

 largest of the four. The basal attachment in all the 

 specimens was at one edge of the thin plate of sponge, 

 and both the broad surfaces were evidently inhalent 

 ones. Very few of the oscula were visible, and those 

 were very minute. The pores were not visible to the 

 unassisted eye, but in portions of the dermis mounted 

 in Canada balsam they were seen to be numerous, but 

 rarely more than one in any of the areas of the dermal 

 reticulations. 



The reticulations of the skeleton are wide and very 

 irregular. The primary lines are multispiculous, and 

 the secondary series are so irregular and numerous 

 that its structure is not readily determined unless 

 under favorable circumstances, in a section carefully 

 cut at right angles to the surface. There is no appre- 

 ciable difference in the size and form of the spicula of 

 the skeleton and those of the dermal membrane, and no 

 other form could be detected in any part of the sponge. 



The only species of known British Isodictya with 



