PLATE XC. 327 



lines of the skeleton, when they reach the inner 

 surface of the dermis, instead of terminating there, or 

 of passing through that organ, are spread out beneath 

 it to some distance in long slender lines, so that when 

 viewed with a low power through the dermal mem- 

 brane, they might be readily mistaken for the fibres of 

 a very slender Desmacidon. This peculiarity in the 

 distribution of its structures, at the first sight, com- 

 plicates their characters to a very considerable extent ; 

 but when once recognised it becomes a valuable dis- 

 criminative character. 



From the extremely irregular construction of the 

 skeleton it is rather difficult to say whether there are 

 any tension spicula on the interstitial membranes ; 

 and although I searched diligently for them I could 

 not find any of the anchorate retentive spicula on 

 them. 



The only species with which I. dubia is liable to be 

 confounded is I. gracilis ; their skeleton structures are 

 very similar, but in the latter the spicula of the primary 

 lines are much stouter than those of I. dubia, and in 

 that species the external form is massive and sessile ; 

 while in I. gracilis it is ramose, and the dermal mem- 

 brane is aspiculous. 



DESMAOTDON EOTALIS, BowerbanJc. 

 Plate XC. 



Sponge parasitical on zoophytes or fuci. Surface 

 even and smooth. Oscula simple, dispersed, minute. 

 Pores inconspicuous. Dermis retiform, rete arranged 

 in hexradiate confluent rotulate areas ; radii compact 

 and multispiculous ; retentive spicula bihamate, simple 

 and contort, few in number, and rarely bidentate, 

 inequi-anchorate, and dentate-palmate, inequi-an- 

 chorate, very minute. Skeleton compact, fibres 

 frequently dividing dichotomously, terminating abruptly 

 on the inner surface of the dermis and dividing to f orm 



