INTRODUCTION. 13 



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. Some- 

 times 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, but Mr. Williamson informs 

 me that he possesses specimens which exhibit bent spines 

 and torn margins, and thus wholly contradict the idea 

 that they were siliceous before they were imbedded in the 

 flint. 



In the present state of our knowledge it would be pre- 

 mature 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. A paper on fossil Xanthidia by Mr. H. H. White, 

 containing descriptions of eleven supposed species, accom- 

 panied by characteristic figures, may be consulted with 

 advantage*. 



In all the Desmidiese, but especially in Closterium and 

 Micrasterias, small, compact, seed-like bodies of a blackish 

 colour are at times met with. Their situation is uncertain, 

 and their number varies from one to four. In their imme- 

 diate neighbourhood the endochrome is wanting, as if it had 

 been required to form them, but in the rest of the frond it 

 retains its usual colour and appearance. I cannot satisfy 

 myself respecting the nature of these bodies, but I believe 

 them either to arise from an unhealthy condition of the plant, 

 or else to be parasitic. 



The only account I have seen of the discovery of fossil 



* Microscopic Journal, vol. ii. p. 35. 



