352 THE MEDALS OF CREATION, Cuar. X. 
slit extending from one cell to another, and opening on the 
umbilicus ; that is, on the side opposite to the spire. There 
are eighteen fossil, and many recent species of this genus. 
Textunaria. Lign. 109.—This, and the following genus, 
belong to that order of Foraminifera in which the segments | 
or cells are arranged in two or three distinct axes (ante, 
p- 342), and bytheir gradual increase give rise to an elongated 
conical but not spiral shell, which in its general outline 
resembles that of certain gasteropoda, but is easily distin- 
guished by its internal structure. The shell is conical, 
compressed, formed of alternate cells, with a transverse 
aperture placed on the inner side. Upwards of thirty fossil 
species are known. The Textularize are in great abundance 
in the cretaceous rocks; and, together with Rotaliz and 
Rosalinee, constitute a large proportion of the minute organ- 
isms of the secondary formations as well as of the present 
Seas. 
VERNEUILINA.—Lign. 109, fig. 3.—A turriculated shell, 
with a slit or aperture transverse to the axis of involution, 
and placed on the umbilicus. This genus, of which but one 
Species is known, is peculiar to the cretaceous deposits. 
STRATA COMPOSED OF FORAMINIFERA.—From this concise 
exposition of the characters of the genera that most frequently 
occur in a fossil state, we pass to the examination of the 
organic composition of those limestones which are in a great 
measure made up of the debris of Foraminiferee. We will 
commence the investigation with that common substance, 
the white chalk of the South-East of England. 
It has long been known that a large proportion of the 
purest white chalk consists of minute chambered shells,* and 
corals. 
Mr. Lonsdale, some years since, first showed that by 
brushing chalk in water, and examining the sediment, shells, 
* Often termed Polythalamia, meaning many chambers or cells. 
