360 



Mr. W. J. Sollas on the Genus Catagma. 



3. {a) Calcareous sponges usually contain triradiate and 

 quadriradiate spicules. 



(b) Siliceous sponges frequently contain quadriradiate, 

 sometimes triradiate spicules [PlectroneUa ; for other instances 

 see my paper on Plectrojiella, in a forthcoming number of the 

 'Annals '). 



4. {a) Calcareous sponges are usually small, and seldom ex- 

 hibit the external characters of Catagma. 



ih) Siliceous sponges often closely resemble Catagma both 

 in size and form. 



Fig. 2. 



Spicule of a Leuconia ( X 435), to indicate the granular manner in which 

 it begins to disintegrate. 



Next, and finally, we have to consider the mineralogical 

 side of the question. Neither Professor Zittel nor myself 

 doubt for a moment that siliceous structm-es may be converted 

 during fossilization into carbonate of lime, and, conversely, that 

 calcareous structures may become silicified : the only difference 

 is as to which process has happened here. 



We may observe, first, that while, on the one hand, we 

 know of a vast number of cases in which siliceous sponges 

 closely allied to existing species have been preserved in the 

 fossil state, on the other hand not a single instance of a fossil 

 calcareous sponge closely allied to any species of our existing 

 seas has yet been described, the supposed fossil calcareous 

 sponges differing widely in structural characters from existing 

 forms. I have not, however, the slightest doubt as to the 

 possibility and even probability of fossil calcareous sponges * 

 being some day discovered. It is useless to allege that cal- 

 careous spicules are too unstable to survive the effects of such 

 a feeble solvent as sea-water j that they soon disintegrate and 

 lose their definite form when mounted in sections of the sponge 



* Protosycon, Zittel, maj- be such a sponge. 



