PREFACE. 



In the first volume of this work, page 159, in the 

 1 Tabular View of Systematic Arrangement,' I included 

 the exotic as well as the native genera ; believing it highly 

 probable that some of the species of the former would 

 before long be found among our British Sponges ; and this 

 idea has been already realised, by my having since that 

 period added two species of Ecionemia and one of Ophlita- 

 spongia, an entirely new genus, to our list of British species. 

 I have followed the same course in the synopsis of the 

 genera in this volume, but to prevent any misconception 

 regarding the British Genera, I have given a list of those 

 which arc described in this volume, with the number of 

 species in each, in the Table preceding the synopsis of the 

 Genera. 



Generally speaking, there is no great difficulty in the 

 determination of the genus, but in some cases more than 

 ordinary caution is necessary in the examination of the 

 specimen under consideration. The greatest difficulty in 

 this respect lies in the determination of specimens belong- 

 ing to the first section of Halichondria, and the first of 

 Isodictya. In the former, the structures are so loosely and 

 irregularly reticulated, as to render it very difficult to dis- 

 tinguish between them, and some specimens of the latter, 



