226 PLATE LXXVI. 



trawlers on the Diamond Ground, off Hastings. It is 

 two and a half inches long, two inches greatest breadth, 

 and one and a half inch in thickness. It has been 

 located on the back of one of the spider crabs, ap- 

 parently from the remains of the carapace in a deep 

 depression on the underside of the sponge, Pisa 

 tetraodon, Leach. Only a small portion of the external 

 surface of the specimen is visible, as it is thinly coated 

 over to a very considerable extent by Hymenacidon 

 lactea, for an account of which I must refer the reader 

 to the description of Plate XXXII, fig. 9, in the present 

 volume. 



I received a single specimen of this species from the 

 Rev. A. M. Norman ; it is a small, irregularly-shaped 

 mass not exceeding three lines in diameter. It was 

 parasitical on a small branched fucus. The oscula are 

 few in number ; some of them rather exceeded a line 

 in diameter. The dermal membrane is evenly spread 

 over the distal terminations of the skeleton lines, and 

 at the first view has the appearance of being furnished 

 with a dermal network, but this appearance arises from 

 its reposing immediately on the terminal portions of 

 the skeleton tissues. 



The skeleton has a strong and compact appearance. 

 The primary lines are usually composed of rarely less 

 than three or four spicula in thickness, and in many 

 of them there are more than can be distinctly counted. 

 The secondary lines have rarely more than one spiculum 

 each. The generic character of the skeleton struc- 

 ture is well preserved in the outer portions of the 

 sponge, but towards its central portions it might 

 readily be mistaken for a Halichondria. Externally, 

 both in its wet and dried state, it might readily be 

 mistaken for a specimen of HaUcnondria incrustans. 



ISODIOTYA IMITATA, BowerbanJc. 



Plate LXXVI. 

 Sponge coating. Surface minutely hispid, even, 



