instead of after, their detachment from it, can scarcely be admitted by the Physiologist as alone justifying an ordinal differentiation, which is not borne out by other structural or physiological diversities.
Having shown in my former paper how completely fallacious is the assumption of M. D'Orbigny that plan of growth affords the key to the natural arrangement of Foraminifera,—any classification that is founded upon it necessarily bringing together generic types which are physiologically most distinct, and separating such as are physiologically most nearly allied,—I shall now confine myself to a concise exposition of what appear to me the principles on which Natural Classification should be founded.
Looking at the Order Reticularia as a whole, the only great physiological distinction at present known to exist among the multitudinous forms of animal life which it includes (our acquaintance with the mode in which the generative function is performed in this group being as yet so imperfect, that no differential characters can be founded upon it), is that presented by the two modes in which the pseudopodia originate, viz.:—either from the surface of the body generally, or from a limited portion of it. The animals of the former type, of which Rotalia may be taken as an example, have a shell whose surface is everywhere perforated with numerous closely set pores; and through these, as observation shows, the pseudopodia extend themselves freely from each of the segments that occupies the subjacent chambers. In those of the latter, of which Miliola may be taken as the type, the walls of the chambers are entirely imperforated; so that the pseudopodia can only issue from the single or multiple aperture, which leads to the last-formed chamber alone. The fundamental importance of this distinction was perceived (as I have already pointed out) by Dujardin; and my own enquiries, which have been pursued on a basis altogether independent of his, have led me most fully to recognize the merit of that far-sighted perception, which would have been more likely to attract the notice it deserved, if its author had been aware that, instead of being isolated from the true Foraminifera by the characters in question, the Miliolæ are really the representatives of that large group of Foraminifera which are distinguished by the porcellanous texture of their shells.
Taking our stand, then, upon the limitation or diffusion of the origin of the pseudopodia—manifested in the imperforation or the perforation of the testaceous envelope,—as a distinction of fundamental importance, we find that the Order Reticularia may be subdivided by this character into two sections; and as it is convenient to base our systematic arrangement of the Foraminifera upon the characters furnished by the shell (though always hearing in mind that these are of value only in so far as they may be taken as exponents of the characters of the animal) these two sections or sub-orders maybe respectively designated Imperforata and Perforata.
In the sub-order Imperforata, the testaceous envelope presents itself under three very different conditions, the membranous, the porcellanous, and the arenaceous; and upon this difference we may
