514 ARTHUR DENDY. 



of the two types and in the conclusions to be drawn therefrom. 

 Hence I propose, at the end of the paper, to give a short sum- 

 mary in which the points of difference and of resemblance 

 between the two genera will be pointed out and discussed. 



Some of the sections were cut in the laboratory of the 

 Zoological Society, at the Zoological Gardens, and others in 

 the laboratory of the Royal School of Mines, South Kensing- 

 ton, and I have great pleasure in taking the present opportunity 

 of expressing my thanks to the authorities of both these in- 

 stitutions, and especially to Professor G. B. Howes and Mr. 

 Beddard, for the great facilities for work afforded to me. 



With regard to the figures illustrating this paper, I feel that 

 a few words of explanation are desirable. None of the figures, 

 excepting those of mere external form and of spicules, are in- 

 tended for exact representations of individual preparations. 

 What I have been unable to make out in one section I have 

 found in others, or vice versa, and the figures are the result 

 of the repeated examination of a large number of prepara- 

 tions. In drawing them, however, the camera has been largely 

 used, and, although they are to a certain extent diagrammatic, 

 I believe them to be far more instructive than any mere 

 facsimile drawings of individual sections could be. Figure 7 

 is a pure diagram showing what I believe to be the arrange- 

 ment of the canal system in Ridleia oviformis, after having 

 repeatedly examined a large number of sections, both transverse 

 and longitudinal. 



In his report on the Hexactinellida of the " Challenger'' Ex- 

 pedition, 1 Professor Schulze, who is undoubtedly the greatest 

 living authority on the anatomy of Sponges, observes : " If I 

 had attempted to copy the individual sections exactly as they 

 appeared, the essential and typical could not, as a rule, have 

 been distinguished from the unessential and accidental, except, 

 of course, by giving a larger number of illustrations than 

 seemed justifiable for such a slight possible advantage," and 

 with this I most heartily agree. 



1 P. 4. 



