XANTHIDIA. 443! 



Near to Sydden Spoiut, and the Roimd Down Cliff, on 

 the Dover beach, Mr. H. Deane cut out a piece of pyrites 

 with the adherent chalk, which, on examination, '' exposed 

 to view bodies similar to, if not identical with, Xan- 

 thidia in flints ; he clearly recognised X. spinoaum, ramo- 

 sum, tuhiferum, simplex, tuhifenmi recurvum, malleoferum, 

 and pyxidiculum, together with casts of Polythalamia, and 

 other bodies frequently found in flints. In shape they are 

 somewhat flattened spheres, the greater pju-t of them 

 having a remarkable resemblance to gemmules of sponge, 

 with a circular opening in the centre of one of the 

 flattened sides. The arms or spines of all appear to be 

 perfectly closed at the ends, even including those which 

 have been considered in the flint, specimens decidedly 

 tubiferous ; showing that if the arms are tubes, they could 

 afford no egress to a ciliated apparatus similar to those 

 existing among Zoophytes. On submitting them to pres- 

 sure in water between two pieces of glass, they were torn 

 asunder laterally, like a horny or tough cartilaginous sub- 

 stance ; and the arms in immediate contact with the glass 

 were bent. Some specimens, put up after several weeks' 

 maceration in water, were so flaccid, that, as the water in 

 which they were suspended evaporated away, the spines or 

 arms fell inclined to the glass. These circumstances alone 

 seem clearly to disprove the idea of their being purely 

 siliceous. The casts of the Polythalamia, portions of 

 minute crustaceans, &c. appeared also to be, like the Xan- 

 thidia, some modification of organic matter ; and in the case 

 of the Polythalamia, the bodies are so perfectly preserved, 

 that in some the lining membranes of the shells are readily 

 distinguishable." 



Mr. Wilkinson, who examined recent Xanthidia found in 

 the Thames mud, and slime, on piles and stones at Green- 

 hithe, said that, in his opinion, they are not siliceous, 

 but of a horny nature, similar to the wiry sponges, which 

 Mr. Bowerbank describes as being very difficult to destroy 

 without the action of fire. He also met with a peculiarity 

 in a X. spinosum, which he has never seen in any other 

 species ; it was in a piece of a gun-flint. There appeared, 

 as it were, a groove or division round the circum- 

 ference; similar to that formed by two cups when placed 



