172 
POPULAE SCIENCE EEYIEW. 
The subjects of this paper appear to be neither the highest 
nor the lowest forms of this Ehizopodous group. The earlier 
observers were misled by the chambered calcareous shells, 
which looked so like those of Nautili that all naturalists 
from Soldani to D^Orbigny fell into one common error 
and called them Cephalopods. To M. Dujardin belongs 
the merit of having put them in their right place, leaving no 
ground for doubt or question respecting their relationship to 
the Amoebae. Starting from this acknowledgment of their 
rhizopodous nature, we may now proceed to notice their more 
important characteristics. These objects require to be studied 
under two aspects, — that of their shells, which are mere 
skeletons, partly external and partly internal ; and that of their 
soft animals, — the living moving organization by which the 
shells are secreted. Owing to their abundance, and the 
readiness with which they may be obtained, the former of these 
have for above a century past attracted the chief, nay the 
almost exclusive, attention of naturalists. We, too, will first 
glance at their chief peculiarities. 
In chemical composition they exhibit two very distinct 
types : first, those in which carbonate of lime is the chief 
ingredient ; and, secondly, those in which sand, cemented 
together by some peculiar animal secretion, is the chief con- 
stituent. But we have also two kinds of calcareous structures. 
In the one which I have elsewhere termed porcelainous, the 
shell is white, semi-opaque, usually shining, exhibiting a rich 
amber-colour when viewed as a transparent object by trans- 
mitted light. Young shells of this class are at first somewhat 
transparent, but they soon assume the characteristic opaque 
aspect of mature forms. 
In the other class of calcareous shells, termed hyaline, the 
transparent appearance is maintained to a much later period, 
and when age has rendered them opaque, they exhibit a dirty 
greenish tint instead of the lustrous whiteness of the porce- 
lainous types ; transmitted fight also fails to reveal the amber 
tint of the latter shells. 
Of the shells in which sand is a constituent element but 
little is known. The animal cement which binds the foreign 
particles together does not appear to be soluble in any alkaline 
solution, as is the case with the sarcode. Treated with a hot 
solution of caustic, the particles lost none of their cohesiveness, 
a fact suggesting a composition analogous to the chitinous 
integuments of insects and crustaceans. There are some forms 
in which this animal substance appears to compose the entire 
shell — unmixed with any inorganic matter. 
Neither of these three varieties of organic composition 
appears to be hmited to particular forms of shells. We find 
