XANTHIDIA. 193 



his Medals of Creation, without any misgiving, had adopted Ehrenberg's 

 ideas concerning them, changed his opinion ; and in his last work re- 

 gards them as having been reproductive bodies, although he is still 

 uncertain whether they are of vegetable origin." 



Ehrenberg and his followers describe these bodies as fossil species 

 of Xanthidium ; but no doubt erroneously, since their structure is very 

 different. For the true Xanthidium has a compressed, bipartite, and 

 bivalved cell; whilst these fossils have a globose and entire one. 



The fossil forms vary like recent Sporangia, in being smooth, bristly, 

 or furnished with spines, which in some are simple, and in others 

 branched at the extremity. Sometimes, too, a membrane may be 

 traced, even more distinctly than in recent specimens, either covering 

 the spines, or entangled with them. Some writers describe the fossil 

 forms as having been siliceous in their living state j but Mr. Williamson 

 informs us that he possesses specimens which exhibit bent spines and 

 torn margins ; and this wholly contradicts the idea that they were sili- 

 ceous before they were imbedded in the flint. In the present state of 

 our knowledge, it would be premature to attempt identifying the fossil 

 with recent species ; it is better, therefore, at least for the present, to 

 retain the names bestowed on the former by those who have described 

 them. 



Near to Sydden Spout and the Round 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, the Xanthidia in flints j . and clearly recognised X. 

 spinosum, ramosum, tubiferum, simplex, tubiferum reeurvum, malleo- 

 ferum, and pyxidwulum, together with casts of Polythalamia, and 

 other bodies frequently found in flints. In shape they are somewhat 

 flattened spheres, the greater part of them having a remarkable re- 

 semblance to some gemmules of sponge, and having 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 to be 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 pressure 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 eva- 

 porated away, the spines or arms fell inclined to the glass. These cir- 



o 



