Page:Natural History Review (1861).djvu/484

472 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.