But though I consider the animal of the Foraminifera as single, and the additional lobes, or segments, as the continuous growth of the same individual, I must state that some eminent naturalists regard the entire structure as a series of distinct individuals, developed by gemmation from the first formed segment, like the clusters of the compound _Tunicata_; and not as a single aggregated organism, made up of an assemblage of similar parts indefinitely repeated. In a palæontological point of view, it matters not which opinion is adopted.[311] [311] See a masterly paper on the structure of Nummulina and Orbitoides, by Dr. Carpenter; Geol. Journal, vol. vi. pp. 21-39, with admirable representations of the structural details. [Illustration: Lign. 109. Foraminifera. _Chalk. Charing._ Fig. 1.--Globigerina cretacea; the original is 1/60 of an inch in diameter. 2.--Textularia Globulosa; 1/40. 3.--Verneuilina tricarinata; 1/30. 4.--Cristellaria rotulata; 1/40. 5.--Rosalina Lorneiana; 1/40. ] [Sidenote: 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_. 5. Segments developed alternately on the right and left of the first, and successively on each side the longitudinal axis; as in _Textularia_, _Lign. 109_. 6. Arrangement of the segments combining the modes of 4 and 5; that is, the segments are formed alternately, but the whole are coiled spirally, either obliquely, or on the same plane; as in _Amphistigena_. 7. Segments round a common axis, on two, three, four, or five, opposite faces, returning after each entire revolution; the new cells being placed exactly on the preceding series; as in _Quinqueloculina_, _Pict. Atlas_, pl. lxii. _fig._ 12.