790 W. J. SOLLA8 ON THE STRUCTURE AND 



43. On the Structure and Affinities of the Genus Siphonia. 

 By W. J. Sollas, Esq., M.A., F.G.S., late Scholar of St. John's 

 College, Cambridge, and Associate of the Royal School of 

 Mines, London. (Eead May 23, 1877.) 



[Plates XXV. & XXVI.] 



1. Introduction. 



Some years ago my friend and tutor Mr. Bonney was kind enough 

 to procure for my examination a collection of phosphatic nodules 

 from the Gault of Folkestone, amongst which were included several 

 specimens of what I considered to be species of SipTwnia in a 

 phosphatized condition. Knowing well, from a previous investiga- 

 tion of phosphatized Ventriculites from the Cambridge Greensand, 

 how perfectly sponge-structure is sometimes preserved in this state, 

 it occurred to me that I had here a good opportunity of determining 

 the minute structure of this sponge and its relations to recent forms. 



Accordingly I requested Mr. Cuttell, of 52 New Compton Street, 

 Soho, to prepare a series of transparent slices from my specimens ; 

 and on examining under the microscope the beautiful sections he 

 made for me I saw displayed, in all its details, the characteristic struc- 

 ture of a Lithistid sponge. I then submitted my preparations to my 

 kind friend Mr. Carter, who not only confirmed my observations, but 

 gave me the benefit of several valuable suggestions, and generously sent 

 me pieces of the recent Discodermia poly discus, Bocage, for comparison. 

 Subsequently Mr. Vicary, with great kindness, put a large number of 

 siliceous specimens of Siphonia from the Blackdown Greensand at my 

 disposal, and I obtained from Mr. Bryce Wright an example of 

 Siphonia {Hcdlirhoa) costata, Lamx., from the Greensand of Wiltshire. 

 Finally my friend Mr. Moore afforded me every facility for the ex- 

 amination of the beautiful collection of Choanites preserved in the 

 Liverpool Free Museum ; and transparent slices taken from speci- 

 mens of that genus were sent me from the Woodwardian Museum, 

 Cambridge, by the courtesy of Professor Hughes. 



After a thorough examination of the material I had thus acquired, 

 my next step was to work out the literature of the subject ; and this 

 proved to be a more laborious and unsatisfactory undertaking than 

 could have been expected. A knowledge of the minute structure 

 of fossil sponges is the first essential towards determining their 

 natural affinities; and of this minute structure pala:ontological 

 works tell us little or nothing. As a general rule, all one can 

 learn from published figures and descriptions relates to external 

 form and coarse structure only; and consequently without a per- 

 sonal examination of the forms described one is not always in a 

 position to determine whether they are rightly assigned to a 

 particular generic group or not. Moreover tho external form of 

 the same species of sponge is often so variable that when one has 



