Page:Natural History Review (1861).djvu/202

190 is either absent altogether, or presents itself under a modified form; and thus we seem justified in the belief that whether either has been derived from the other, or both have been derived from some intermediate form (such as that which seems common alike to the young of both), the modifications which have given rise to the marked differences they now exhibit, are mainly due to diversities in the external conditions under which they have been respectively propagated.

But to what other type does Peneroplis itself present the closest approximation? By systematists in general, the intimate relationship which I have shown it to possess to the helical type of Orbiculina has been so slightly regarded, that it has been considered as at least equally related to the Operculina type; and yet, as I shall presently show, these two types are removed from each other in all the most essential features of their structure, as far as any two polythalamous Foraminifera can be. And the idea of the derivation of Peneroplis from the same stock with Orbiculina seems justified by the fact that the young forms of the two are frequently so alike as not to be distinguishable by external characters alone, whilst their internal difference consists only in the presence or the absence of the secondary or transverse septa—a character which I have shown reason to regard as variable in this group.

Notwithstanding, therefore, the apparently wide divergence of the cyclical Orbitolites, the helical Orbiculina, the fusiform Alveolina, and the simply-chambered Peneroplis and Dendritina, these several types must be regarded as most intimately related to one another; and that relationship seems to me much more likely to have arisen from a common ancestral descent, than from the original creation of independent types, capable of graduating into each other so continuously as almost to assume each other's characters.

It is very important to remark that they all possess that peculiar texture of shell, which is designated by Professor Williamson as porcellanous; presenting an opaque white hue when seen by reflected light, but a rich brown or amber colour, when seen by light transmitted through thin natural lamellæ or artificial sections. This substance is entirely structureless, and possesses no great density or tenacity. Moreover, in all the foregoing types, each of the septa intervening between the chambers consists of only a single layer; and the passages of communication between them are, for the most part, so large and free, that the segments of the sarcode-body are but very imperfectly isolated from each other; and, as might be anticipated from this incompleteness of separation, it is here that variations in the mode of communication between the chambers seem to be of least account. It is in this type that we re-