4.--Portion of a Nautilus, showing five chambers, partially separated, each pierced by the siphunculus: in Flint, from Ireland. 5.--The body of a Rotalia, in Flint; the shell is not apparent. ] [Sidenote: ROTALIA--ROSALINA.] Rotalia. _Lign. 114._--The shell, though nautiloid in its contour, is regularly turbinated, the cells not globular; the last cell has a central, semilunar, transverse, aperture. There are fifty fossil species. The Rotaliæ appear in the Lias, Oolite, and Chalk, in immense numbers, and swarm in the present seas. Rosalina. _Lign. 109, fig. 5._--The shell is depressed; the spire apparent on one side; the aperture is a prolonged 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. Textularia. _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 by their 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 distinguished 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 Textulariæ are in great abundance in the cretaceous rocks; and, together with Rotaliæ and Rosalinæ, constitute a large proportion of the minute organisms 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. * * * * * [Sidenote: CHALK ANIMALCULITES.] 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 Foraminiferæ. 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,[322] and corals. [322] Often termed _Polythalamia_, meaning many chambers or cells. Mr. Lonsdale, some years since, first showed that by brushing chalk in water, and examining the sediment, shells, corals, and foraminifera might be obtained in abundance; but it was not at that time suspected that the residue of the detritus was almost entirely composed of distinct organic structures, so minute as to require high magnifying powers, and a peculiar mode of manipulation, for their detection and definition.