PllEFACE. IX 



sectibns, determinable by the forms of the spicula; and 

 each of these sections are again divisible, into those which 

 have the spicula smooth, and those which are furnished 

 with spines. I have considered the sections and sub- 

 sections into which the genera are thus divided as per- 

 manent divisions, applicable to all the genera alike; the 

 sections being distinguished by one, two, three, or more 

 asterisks, and the sub-sections are indicated by letters. 

 The species in some of these divisions and subdivisions are 

 already very numerous, as at the present time we have 

 in the genus Hymeniacidon 42, in Halichondria 28, 

 and in Isodictya 43 species ; and I believe these divisions 

 and subdivisions to be the more necessary, as from 

 the quantity of new species I have lately received, there 

 is every appearance that the number of British Sponges 

 will be greatly multiplied by the researches of naturalists 

 before many years shall have elapsed. 



I have not adopted the extensive list of synonyms of the 

 British species given by Dr. Johnston, as I have good 

 reason to believe that many of them are unavoidably very 

 doubtful ; I must therefore beg to refer the reader, who 

 may wish to enter upon an investigation of that portion of 

 their history, to that learned author's ' History of the 

 British Sponges,' &c. Thus the references to habitats of 

 Hymeniacidon Jicus, carnosa, and suherea, are rendered 

 very uncertain, from the habit each has of simulating the 

 forms assumed by the other two, and also from species 

 having hitherto been established from external form only, 

 while each individual varies exceedingly in that character 

 at different periods of its development, and in accordance 

 with the necessities induced by the peculiarities of its 

 locality. The wide diffusion of the species renders it un- 



