THE EXCAVATING SrONGES. 339 



Now, I have numerous slides so mounted of Cliona celata, Grant, 

 which species is undoubtedly the same as Johnston's second va- 

 riety under that specific denomination, and I have never found on 

 any of them more than one form of spiculum, or any of the forms 

 that characterize Cliona Northumbrica, C. gracilis, C. Howsei, 

 C. Alderi, C. corallinoides, C. lobata or C. vastifica; neither have 

 I in any of these species found the exact form of spiculum that 

 distinguishes C. celata. In fact the habit of the sponge, and 

 the characters of the spicula are so well pronounced, that with 

 the exception of one, which I admit to be critical, few natural- 

 ists, after a careful examination of the species I have described, 

 will doubt their distinctness. 



And here it may be observed, that the study of C. celata alone 

 is not sufficient for the full comprehension of the questions con- 

 nected with the excavating sponges. This species is not by any 

 means typical of the group ; it has but one form of spiculum, 

 while by far the greater number have two or three kinds, and 

 the sponge itself does not assume, in a decided manner, that 

 lobed structure which is so dominant in the genus. All the 

 British forms should be carefully examined ; and the foreign, 

 which are very abundant, should be investigated with equal 

 assiduity. "When this is done, there will be little difference of 

 opinion on most questions connected with the subject. 



"When I drew up the descriptions of the species I had not 

 mounted any of the spicula in balsam ; it was not till sometime 

 afterwards that this was done, and the discovery made that a 

 third minute form of spiculum had been overlooked in several of 

 the species. It was unfortunate that this had escaped observa- 

 tion in the first instance, as it aids materially in distinguishing 

 the species, though the characters originally given appear amply 

 sufficient for the purpose. 



Cliona celata possesses only pin-like spicula according to all 

 writers on the subject, and it is equally true that most of the 

 Clionce have likewise pin-like forms ; but in very many instances 

 they have also associated with them other forms ; and it is such 

 association of various kinds of spicula that chiefly characterizes 

 the species, and that distinguishes most of those I have described 



