the helical plan, that if such had been collected before their assumption of the cyclical mode of growth, their essentially Cyclostègue character would not have been suspected.
Again, I have shown (2nd series) that a parallel variation is displayed by the genus Orbiculina, whose ordinarily helical plan of growth has caused M. D'Orbigny to range it among his Helicostègues, notwithstanding that in fully developed specimens its mode of growth is not unfrequently cyclical. The occasional exchange in this type of one plan of increase for the other, at an advanced period of life, is a fact of very high interest; for when an Orbiculina has undergone this change, the outer or cyclical portion of its disk can in no way be distinguished from that of Orbitolites; and the only difference between these two types which has any semblance of validity is the absence in Orbitolites of those successive encasings of the central nucleus, the presence of which seems to be a constant feature in Orbiculina.
It is to be observed, however, that these successive encasings are due entirely to the extension of the later whorls of the spire over the earlier, and that they are no longer found in Orbiculina when the helical mode of growth gives place to the cyclical. Hence it seems not unfair to surmise that if the helical growth of an aberrant Orbitolites were to continue until its spire had made several turns, instead of stopping before the completion of one, its nucleus would receive successive investments from successive whorls, just as in the typical Orbiculina, and the only difference between these two types would thus vanish.
On the other hand, if the helical growth of an Orbiculina were to give place to the cyclical at an unusually early period, the central nucleus would receive no investment, and would present the flatness by which that of Orbitolites is characterised as compared with that of the typical Orbiculina. Hence the idea of the derivation of Orbitolites and Orbiculina from the same original must be admitted to be scarcely less probable than that of the derivation of the helical and the cyclical forms of Orbiculina, or of the simple and complex types of Orbitolites, from a common parentage.
Let us now apply the same mode of inquiry to Alveolina. I have shown (2nd series) that this organism is closely allied in every other respect than its geometrical plan of growth to the types we have just been considering; the structure of the shell and its relations to the contained body, and the relations of the segments of that body to each other, and to the external world, being essentially the same in them all.
Now however improbable it may seem at first sight that an Orbitolites, which extends itself as a flat or bi-concave disk by successive concentric growths, and an Alveolina acquiring a fusiform shape by successive turns round a progressively elongating axis, should have a common original, yet, when the intermediate links are duly studied, a continuous gradation is found to be established. For, as has just been shown, a longer continuance of the helical mode of growth in which Orbitolites often commences, would really produce an Orbiculina, with its centre so invested by successive whorls as to form a vertical
