A very remarkable point which has led to great confusion in the study of the Foraminifera, is the fact that the shell on which we base our characters of classification, may vary very much, even within the same individual. Thus in the genus _Orbitolites_ the first few chambers of the shell have the character of a Milioline, in _Orbiculina_ of a _Peneroplis_. The arrangements of the Milioline shell, known as Triloculine, Quinqueloculine, and Biloculine respectively, may succeed one another in the same shell (Figs. 14 4, 15). A shell may begin as a spiral and end by a straight continuation: again, the spherical _Orbulina_ (Fig. 16 1) is formed as an investment to a shell indistinguishable from _Globigerina_, which is ultimately absorbed. In some cases, as Rhumbler has pointed out, the more recent and higher development shows itself in the first formed chambers, while the later, younger chambers remain at a lowlier stage, as in the case of the spiral passing into a straight succession; but the other cases we have cited show that this is not always the case. In _Lagena_ (Fig. 13 2) the pylome is produced into a short tube, which may protrude from the shell or be turned into it, so that for the latter form the genus _Entosolenia_ was founded. Shells identical in minute sculpture are, however, found with either form of neck, and, moreover, the polythalamial shells (_Nodosaria_, Fig. 13 3), formed of a nearly straight succession of _Lagena_-like chambers, may have these chambers with their {67}communications on either type. Rhumbler goes so far as to suggest that all so-called _Lagena_ shells are either the first formed chamber of a _Nodosaria_ which has not yet become polythalamian by the formation of younger ones, or are produced by the separation of an adult _Nodosaria_ into separate chambers. [Illustration: FIG. 15.—A, Megalospheric; B, microspheric shell of _Biloculina_. _c_, The initial chamber. The microspheric form begins on the _Quinqueloculina_ type. (From Calkins' _Protozoa_.)]