CLASSIFICATION OF THE FORAMINIFERA.
Classification of the Foraminifera.—The number of genera is so great, that I can only attempt to convey a very general idea of the principles of classification adopted by M. D'Orbigny, and give a few illustrations of some of the most abundant fossil species.
That the reader may be cognizant of the usual aspect of these shells five specimens from the Chalk, belonging to as many genera, are here represented ([Lign. 109]); the deposit whence they were obtained will be described hereafter.
As the mode in which the growth of the body, and consequently of the shell, takes place, differs greatly in certain groups, an obvious and natural arrangement is suggested, by which the class is divided into seven orders:—
1. The primary, or simplest type; one segment or cell; as in Orbulina.
2. The segments arranged in circular lines; as in Orbitolina.
3. Segments disposed in a straight or arched single line; successively increasing from the first to the last cell; as in Nodosaria, [Lign. 111].
4. Segments, spirally and discoidally disposed, on the same plane, like cells of the Nautilus; as in Cristellaria, [Lign. 109].
The same type, but coiled obliquely, and inequilaterally, like the shells of Gasteropoda; as in Globigerina, [Lign. 109], Rosalina, [Lign. 109].