ADDITIONS. 355 



ISODICTYA DENSA, Plate L. 



Among the sponges sent to me for examination from 

 the Liverpool Museum, there was a box containing 

 numerous fragments of this species. They were in an 

 excellent state of preservation, and in every character 

 were in perfect unison with the type-specimens from 

 Polperro by Mr. Norman. 



ISODICTYA HYNDMANI, Bowerbank. 

 Halichondria JHyndmani, Plate XLVI. 



From the very thin deposit of this sponge on the 

 shells of Pecten opercularis, on which it is usually found, 

 and from the complicated nature of its growth, it is 

 very often diffcult to determine the genus, as unless 

 the specimen of an Isodictya be viewed in- a section at 

 right angles to its surface, it is very apt to be mis- 

 taken for a Halichondria, and this has been my case in 

 describing this species, in the first instance, from the 

 limited number of specimens in my possession. In 

 re-examining a considerable number of fresh ones in a 

 much finer condition than those on which I founded the 

 species, Halichondria Hyndmani, I have been irresist- 

 ibly led to the conclusion that the species is truly an 

 Isodictya. A certain amount of alteration in the 

 specific description will, therefore, be necessary, and I 

 propose that the following should be substituted for 

 the original one, page 264, Vol II. 



ISODICTYA HYNDMANI, BowerlanL 



Sponge. Thinly coating or branching, and an- 

 astomosing; branches issuing from small attenuated 

 bases, slightly compressed, terminations corymbose. 



