See 
INHABITING MULTILOCULAR SHELLS. 461 
considered sufficient ground to separate and form them into a sub- 
genus. 
In the second section, where the shell is discoidal, but the whorls 
of the spire visible, the most interesting genus is that which has 
been called ‘‘ Lenticulites’—the ‘‘ Operculina” of D’Orbigny. It 
is common among the fossils in the neighbourhood of Bordeaux, 
Dax, &c., and bears externally considerable resemblance to an Am- 
monite. 
The third section differs from the second very much as the Nau- 
tilus differs in appearance from the Ammonite: the whorls of the 
spire successively embrace all but the last, and so none but the last 
is visible. The fossil species of Spirolinites, Cristellaria and Num- 
mulites, all belonging to this division of the group, have long been 
looked upon as the most important among the fossil Foraminifera. 
They are in number about thirty; but the individuals of these 
thirty species are so widely distributed and so singularly abundant, 
especially in the case of the Nummulite, that whole mountains are 
made up of them, and in some countries their remains form no in- 
considerable portion of the solid masses of limestone used for building. 
It may be quoted, as an instance of this, that some of the great pyra- 
mids of Egypt are built of “Nummulite” limestone—in other 
words, are chiefly made up of little lenticular shells, few of which 
are so large as a half-erown piece, while by far the greater number 
vary in size from that of a small pin’s head to about three quarters 
of an inch in diameter. 
The Spirolinite is a genus at present only known from fossil spe- 
cies, which bear some resemblance to the siphoniferous species, Spi- 
rula and Lituite, the spire projecting, at a certain age, in a straight 
line, and then forming a cylindrical tube. In this, as in many other 
genera, the septum of the young individual is provided with several 
apertures, although afterwards the number diminishes ; and when 
the animal has attained a certain age there is only one to be ob- 
served. 
The Cristellaria occur both fossil and in a recent state. In ex- 
ternal configuration, the shells of this genus may be compared to 
the Argonauta, since the point of the depressed spire is eccentric ; 
but, like all the other Foraminifera, they are divided into a multi- 
tude of little cells opening into each other, though without any si- 
phuncle passing through them. The fossil species are chiefly from 
the Coroncine (Italy), although seveval are found in the limestone 
at Caen. 
Of the Nummulite we have already spoken. Its remains abound 
