NOTICES OF BOOKS. 505 
gemmæ, in which however we should consider the function of 
the latter to be that of multiplying the individual plant, rather 
than of reproducing the species, 
The following extracts (pages 12 and 13,) will be interesting 
to the Geologist, since they relate to bodies coming under his ob- 
servation not unfrequently, and respecting the nature of which 
much uncertainty has been felt, though the matter seems now, by the 
researches of Mr. Ralfs and others, to be set completely at rest:— 
“That the orbicular spinous bodies so frequent in flint are fossil 
sporangia of Desmidiee cannot, I think, be doubtful when they 
are compared with figures of recent ones. Indeed one celebrated 
geologist, Dr. G. Mantell, who, in his “ Medals of Creation,” 
without any misgiving, had adopted Ehrenberg’s ideas concerning 
them, has changed his opinion, and in his last work regards them 
as having been reproductive bodies, although he is still uncertain 
whether they are of vegetable origin. Ehrenberg and his fol- 
lowers 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 fos- 
sil 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 cover- 
ing the species 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 contra- 
dict the idea that they were siliceous before they were imbedded 
in the flint.” 
We must not omit to bear testimony to the great beauty a 
perfect accuracy of the drawings which illustrate the work; they 
do credit to the pencil of the Author’s able coadjutor Mr. Edward 
Jenner, whose valuable assistance has been fully and properly ac- 
knowledged by Mr. Ralfs in the preface to the volume: and we 
have great pleasure in taking this opportunity of expressing the 
