segments much more independent of one another, than they are in the porcellanous type; and their isolation is marked by these two important peculiarities in the structure of the shell,—first, that each segment has its own complete wall, so that the septa between successive chambers are double,—and second, that the apertures of communication through the septa are far smaller than in porcellanous shells, as is seen in comparing a Vertebralina or Miliola with a Nodosaria or Cristellaria, or, in the unilocular types, on comparing the aperture of a Gromia with that of a Lagena. It is in this type alone that we meet with an "intermediate skeleton" nourished by a "canal system" that is connected with the cavities of the chambers; although this feature is wanting in the lower types of the series, yet its presence in the higher, most strongly differentiates them from the forms of the porcellanous type to which they bear the closest resemblance. In certain genera of this as of the porcellanous series, we find the surface of the shell occasionally roughened by the adhesion of arenaceous particles; but these are imbedded in true shell-substance, which is never wanting; and as the very same forms may be altogether free from arenaceous deposit, its presence is obviously not essential but is (so to speak) accidental, and constitutes no ground for even specific distinction.
As the texture of the shell throughout the whole of this series is essentially the same,—the variation in the diameter of its tubuli being the only difference of any mark,—we have not the same easy means of subdividing the Perforated group into families as we possess in the case of the Imperforate; and this division must consequently be based on the aggregate of characters supplied by the coarseness or firmness of the tubuli, the mode of communication between the chambers, and the general plan of growth. To enter into details upon these points would be foreign to my present purpose, which has been merely to set forth the general results at which I have arrived; and these I now offer to the criticism of such Naturalists as interest themselves in the study of the group to which they relate.
tubuli; and it comes to be a very interesting inquiry what relation there may be between these two substances as to the mode of their formation. There is reason to consider the shell-substance of the Foraminifera as an excretion from the protoplasmic mass of which the body itself is composed; just as the cellulose wall of the vegetable cell, which may be consolidated by carbonate of lime (as in Corallines) or by silex (as in Diatoms) is an excretion from the contained endochrome. The new lamellæ of shell successively added to the external surface of the preceding, in cases in which the spiral lamina of each new whorl completely invests the old, would block up its pores, if the continuity of the tubuli were not maintained by the extension of the pseudopodia through the freshly consolidating substance, and this, by moulding itself upon the pseudopodia that issue from the orifices of the subjacent surface, will itself be rendered tubular, and will continue to allow the passage of the pseudopodia from the earliest chambers through the last formed layer of shell. And I would suggest it as a subject for inquiry whether in the formation of dentine and other calcified tubular tissues of higher animals, the tubular structure is not really the result of the consolidation of an excretion-substance around filamentous prolongations of the active protoplasmic substratum from which it is exuded.
