OF THE FAMILY BACILLARIA. 119 



SPHERASTRUM. 



Free^ a simple carapace, univalve, smooth, injlated, forming 

 groups of various forms hy imperfect spontaneous division. 

 I have not yet detected any specimens of this genus. 



XANTHIDIUM. 



Free, carapace simple, univalve, globular, hristling ivith points 

 or setce, isolated, binary or quaternary, {catenate ?) 



Tliis genus is very interesting, from the fact that bodies, al- 

 most identical in form with the living species, occur abundantly, 

 preserved in the fossil state in flint. Drawings of several fossil 

 species may be seen in the Annals of Natural History, PI. 9, 

 accompanying an interesting paper on the Organic Remains in 

 the Flint of Chalk, by the Rev. J. B. Reade. The directions 

 given for finding these bodies in flint, are " to chip off thin frag- 

 ments which may be attached by means of Canada balsam to 

 slips of glass, and then coated on the outer sm-face with hard 

 spirit varnish. A hundred specimens may be thus cut, poUshed, 

 and mounted for the microscope without trouble and expense, 

 and in less time than an expert lapidary could prepare a single 

 slice with the diamond mill and polishing tool." Many of the 

 common gun flints contain these bodies. I am indebted to 

 E. J. Quekett, Esq. of London, for very fine specimens of fossil 

 Xanthidia ; among them is a slice of flint prepared by a lapi- 

 dary, which contains in a space of less than a square inch, as 

 many as eight or ten very perfect Xanthidia, a scale of a fish, 

 and other organic bodies. 



There has been much discussion as to the real nature of the 

 bodies in flint which so closely reserhble the recent Xanthidia. 

 Turpin mistook them for eggs of Cristatella, to which however 

 they have but little resemblance. Ehrenberg gives figures in 

 his small treatise, "die Fossilen Infusorien und die lebendige 

 Dammerde," PI. I, figs. 2, 12 to 17, which show the closest re- 

 semblance between the recent and the fossil species. It is 

 remarkable, however, that the recent species are inhabitants of 

 fresh water, while flint is undoubtedly of marine origin. I have 

 not seen distinct motion in any of our species. 



