
432 REVIEWS. 
siderable thickness are found, on examination, to be made 
up almost entirely of thcir silicious skeletons. Thus in the 
chalks and marls of Sicily and Greece, Ehrenberg detected 
vast numbers of forms ; and at Oran, in the north of Africa, 
is an extensive stratum made up of the remains of Polyeis- 
tinez and similar organisms, both animal and vegetable. 
The famous infusorial strata of the States of Virginia and 
Maryland on our Atlantic coast, and of California on the 
Pacifie, have, mixed with the minute plants known as Diato- 
mace, many very fine species of Polycistinez, as well as the 
remains of sponges. The most remarkable deposit, how- 
ever, of this character is that which makes up the greater 
part of the island of Barbadoes. This rock is, in many 
places, almost entirely formed of these glassy shells. The 
materials which led to this discovery, in the year 1846, were 
furnished by the geological researches of Sir. R. H. Schom- 
burgh, henee one of the most beautiful species has been 
nested after him. 
The variety of form wid outline which the Polycistinee 
assume is very great, and always of great beauty and grace, 
While their minute dimensions make them, if possible, still 
greater sources of admiration to the student of nature who 
thus finding strata of rocks, of considerable thickness, made 
up of their delicate remains feels the truth of the words of 
the poet, when he says.— 
i * The dust we walk upon was once alive.? 

REVIEWS. 
THE METAMORPHOSIS OF CnaBs.*— That insects undergo a metamor- 
phosis was known by the ancients; the discovery that crabs and worms 
undergo a true metamorphosis, scarcely less striking than that of insects, 
is not more than thirty years old. The Nauplius form, here figured, 




Was known to naturalists in the days of O. F. Müller (who wrote à 
*Fur Darwin. Von Fritz Maller. With ven woodents, Leipzig, 1864. 8vo, pp. 9l. 
sixty-se 
Also recently translated translated and published in London by Van Voorst. 
