AFFINITIES OF THE GENUS SIPHONIA. 835 



garded as a great boon to palaeontologists ; but at the same time he 

 thought there was some difficulty in ascertaining the structure, and 

 in recognizing Siphonice by their structural characters. He inquired 

 of the author what he regarded as the distinguishing characters of 

 the genus Siphonia. He referred to the sponges of the chalk of 

 Yorkshire, which he found to be siliceous by treating them with 

 acid. 



Mr. Hulke inquired of the author what was the nature of the 

 triradiate cavity seen in the spicules with botryoidal branches. 



Prof. Jtjdd referred to the discovery of liadiolarians in Carboni- 

 ferous rocks near Chester, and stated that, on dissolving portions of 

 the rock which clearly show the Radiolarian structure, the latter 

 entirely disappears, but at the same time the rock itself furnishes 

 small crystals of quartz. This seemed to be confirmatory of Mr. 

 Sollas's statements. 



Prof. Tennant stated that the late Dr. Bowerbank's collection of 

 fossil Sponges, including many microscopical sections, is now in the 

 British Museum, and that a fine series of Greensand fossils, con- 

 taining many Sponges, collected by the late Mr. Bensted, is in the 

 Museum at Maidstone. 



The President asked the author whether he knew of any other 

 cases of replacement of silica by calcareous matter. He suggested 

 that the supposed lladiolaria of the Carboniferous rocks of Cheshire 

 might possibly belong to somewhat lladiolarian types in which the 

 solid parts were calcareous instead of .siliceous. So far as he knew, 

 there were no instances of the replacement of silica by carbonate 

 of lime in the Padiolaria of the Barbadoes earth. 



The Author, in reply, said that the Yorkshire sponges mentioned 

 by Mr. Charlesworth are often siliceous externally and calcareous 

 within. The invention of their treatment with acid was rather an 

 unfortunate one, as this treatment often removes delicate surface 

 spicules, and thus modifies the external characters. The form of 

 spicules which he had described as possessing three botryoidal arms 

 were peculiar to the recent sponge Discodermia and to Siphonia, and 

 would suffice to distinguish the genus. The triradiate cavity shown 

 in the head of these spicules represents a triradiate canal. He 

 stated that he had seen Radiolarian forms from the Carboniferous 

 deposits of Scotland among the spicules which had been detected by 

 Prof. Young. In Stromatopora, which he held to have been origin- 

 ally siliceous, the silica was converted into carbonate of lime, and 

 similar crystals of silica to those already referred to were found here. 

 The silica of the organism and that of quartz were in very different 

 conditions as regards solubility ; and the former contains organic 

 matter. He thought that when remains were found presenting 

 precisely the characters of living organisms we were bound palaeon- 

 tologically to regard them as of the same nature ; and after refer- 

 ring to various instances cited, he said he thought the case of the 

 substitution of siliceous by calcareous matter might be regarded as 

 fully established. 



