Vlll PREFACE. 



without the section for examination be very carefully made 

 from a specimen in a good state of preservation, and 

 correctly at right angles to the surface ; but in all cases of 

 the examination of a specimen, the last observation should 

 be especially impressed on the memory. 



In the synopsis of the Genera I have endeavoured to give 

 the student every facility for references to the letter-press 

 and plates of the first volume of this work, and this is the 

 more necessary, as the terminology adopted in the generic 

 and specific descriptions is to a great extent new to this 

 branch of Natural History. This facility of reference will, 

 I trust, aid the student materially in his examinations of 

 both genera and species that may be new to him. 



Notwithstanding my long and intimate acquaintance with 

 the species of British Sponges, such is their extreme 

 variations in colour, size, and form, especially in the dried 

 state, that it has frequently occurred that I have failed to 

 recognise specimens of some of the commonest species, 

 until I have submitted them to a regular microscopical 

 examination ; and indeed there are some that so closely 

 resemble each other, in all their external characters and 

 habits, as to render such an examination absolutely neces- 

 sary to the correct determination of the species. I have 

 therefore deemed it necessary, not only to render the genera 

 as distinctly recognisable as possible, but also when the 

 number of species in a genus are numerous, to again 

 divide and subdivide them by means of the peculiarities of 

 the spicula of the skeleton, so as to facilitate the labour of 

 the determination of the species within the smallest possible 

 range, for the convenience of the student. To effect this 

 purpose, I have divided, when necessary, each genus into 



