INTRODUCTION. 



" THE third volume of this work was scarcely finished 

 when the Rev. A. M. Norman, who had already rendered 

 me such important service by placing his very fine 

 collection of Sponges in my hands for examination and 

 description, sent me a large quantity of additional 

 specimens which he had recently acquired during his 

 dredging excursions, and in other ways. It is on this 

 collection that the present supplemental volume is 

 chiefly based, but I am also indebted for other new 

 species to my old friend Mr. Peach, and to Mr. Hillier 

 of Ramsgate. 



" The greater part of the Sponges sent to me by Mr. 

 Norman were from the North-West of Ireland, and 

 these were found to be especially rich in representatives 

 of the genus Isodictya. The very numerous species of 

 this genus already published rendered the labour and 

 care necessary for the examination and comparison of 

 the newly acquired species so great, that I soon felt 

 the necessity of devising a series of divisions and sub- 

 divisions of the known forms to facilitate the exami- 

 nation of those which I had now to determine. These 

 divisions and subdivisions proved so very useful, and 

 the discrimination of the species was found to be so 



VOL. iv. 1 



