linear axis; and we find this axis in Orbiculina, sometimes equalling in length the diameter of the spire, so that this organism at an early stage of its growth may be nearly spheroidal. Now among the various types of fossil Alveolina, there are some whose shape, instead of being fusi-form, like that of the recent type I have described, is almost identical with that of a spheroidal Orliculina; and the general structure of two such organisms will be so nearly identical, that I cannot see any difficulty in referring them to a common original. And when we examine a series of such fossil types, we see that the}' present a wider and wider divarication from the Orliculina type in this one particular alone, that whilst the later growth of Orliculina tends to liken it to Orbitolites, that of Alveolina tends to the continual elongation of its vertical axis—a difference which all analogy would indicate to be one of far too small account in this group to be justly taken as a ground of original distinction.
In the assemblage of forms which I have thought myself justified in re-assembling under the designation Peneroplis (3rd series), we encounter other remarkable series of variations, the principal of which have given occasion to the formation of the two additional genera Dendritina and Spirolina. With an exceedingly close conformity in the texture and in the superficial markings of their shells, as well as in their general plan of growth, we observe a marked diversity in the form and proportions of the spire, especially in the later stages of its growth, and a still greater divergence in regard to the form and disposition of the septal apertures. For in the type to which M. D'Orbigny restricts the generic designation Peneroplis, we usually find the spire rapidly widening and becoming proportionally compressed in each succeeding convolution; whilst in that which he distinguished as Dendritina, the spire widens but slowly whilst increasing rapidly in turgidity. Further, in the one type, as in the other, the later extension is often in a straight line, instead of continuing to follow the spiral course; and on this variation alone, which (as will presently appear) is of no account whatever among Foraminifera, has been erected the genus Spirolina. Now, in the typical Peneroplis, the septal plane presents a single linear series of minute rounded pores, whilst in the typical Dendritina we find in their place a single large orifice with radiating extensions, the difference between these two modes of communication being as great as we find between al- most any two types of Foraminifera whatever. Yet I believe that no one who will go through the details of the evidence I have collected from the study of transitional forms, will have any doubt that Peneroplis and Dendritina may have had a common progenitor, and that the peculiarity in the mode of septal communication that characterises each is intimately related to the compressed or turgid form of the spire in each case; whilst the different forms of Spirolina type, among which we find the most remarkable transitional conditions of aperture, are so obviously related to one or other of the foregoing, that no reasonable doubt can exist of their derivation from these. Now, the geographical distribution of the two fundamental types is so far different, that where one prevails, the other
