Mr. Derby's specimens contained the curious spiral sea-weed known as _Spirophyton_, and also minute rounded Sporangites like those obtained in the Erian of Ohio, and of which specimens had been sent to me some years before by the late Prof. Hartt. But they differed in showing the remarkable fact that these rounded bodies are enclosed in considerable numbers in spherical and oval sacs, the walls of which are composed of a tissue of hexagonal cells, and which resemble in every respect the involucres or spore-sacs of the little group of modern acrogens known as Rhizocarps, and living in shallow water. More especially they resemble the sporocarps of the genus _Salvinia_. This fact opened up an entirely new field of investigation, and I at once proceeded to compare the specimens with the fructification of modern Rhizocarps, and found that substantially these multitudinous spores embedded in the Erie shales may be regarded as perfectly analogous to the larger spores of the modern _Salvinia natans_ of Europe, as may be seen by the representation of them in Fig. 16.
[Illustration: Fig. 16.--_Sporangites_ (_Protosalvinia_). A, _Sporangites Braziliensis_, natural size, AX, Same, magnified, B, _Sp. biloba_, natural size, C, Detached macrospores. D, Spore-cases of Salvinia natans. DX, Same, magnified. E, Shale with sporangites, vertical section, highly magnified.]
The typical macrospores from the Erian shales are perfectly circular in outline, and in the flattened state appear as discs with rounded edges, their ordinary diameter being from one seventy-fifth to one one-hundredth of an inch, though they vary considerably in size. This, however, I do not regard as an essential character. The edges, as seen in profile, are smooth, but the flat surface often presents minute dark spots, which at first I mistook for papillæ, but now agree with Mr. Thomas in recognising them as minute pores traversing the wall of the disc, and similar to those which Mr. Newton has described in Tasmanite, and which Mr. Wethered has also recognised in the similar spores of the Forest of Dean shales. The walls also sometimes show faint indications of concentric lamination, as if they had been thickened by successive deposits.