2 BRITISH SPONGIAIX2E. 



greatly facilitated by their use, that I have been 

 naturally led to the conclusion that they would, if 

 published, be of similar assistance to my brother 

 naturalists who may take up the study of these in- 

 teresting animals ; and as other genera, such as Hyme- 

 niacidon and Halichondria, also contain a very con- 

 siderable number of species, I have applied to them the 

 same system of division and subdivision. 



" In the preface to Vol. II (p. viii), I had already 

 dilated on the necessity of the subdivision of the species 

 of each genus in accordance with the forms and other 

 peculiarities of the skeleton spicula so as to facilitate 

 the recognition of the species ; and as our knowledge 

 of the British Sponges has advanced and the number 

 of known species greatly increased since 1866, it has 

 now become necessary, as I have just stated, to still 

 further subdivide the species in those genera in which 

 they have become inconveniently numerous. I have, 

 therefore, in the tabulated list of Sponges which follows 

 these introductory remarks adopted a mode of sub- 

 sections in many genera by the use of the characters 

 afforded by the dermal membrane in conjunction with 

 those of the skeleton. 



" The accurate discrimination of the numerous and 

 protean animals which constitute the class of the 

 Spongiadse is still a growing science, and as we advance 

 in our experience of the variable characters, from 

 diversities of habitat and other causes, not only of 

 the different genera and species, but also of the in- 

 dividuals of the same species, we find that their correct 

 determination necessitates a more and more minute 

 investigation into the peculiarities of their anatomical 

 structures. The genus in some few cases may be 



