Page:Popular Science Monthly Volume 16.djvu/479

 abundance. In the Cambrian and Silurian, fossils are found that are differently interpreted, and in which at present some think they see algæ. The famous bilobites, so abundant at the base of the Silurian, appear also to have been algæ of very great height. Finally, certain marine plants, as those that are represented by Fig. 3, are connected with a type of algæ so marked that it is difficult to mistake them.



Many of these plants are undeniably linked with more modern types, of which they bear the generic form, and prove that this primordial flora is not really separated from that which followed it. We can even affirm that certain Silurian algæ have had a duration so prodigious and a tenacity of character so pronounced that their last direct descendants were living in the European seas in the middle of Tertiary time. As to primordial land-plants they are excessively rare, and those that we have gathered seem to demonstrate that in the Silurian epoch to which they belong the vegetable forms represented types that we encounter in subsequent formations, and that are characteristic. In Fig. 4 are shown those that M. Lesquereux observed in the Upper Silurian of the United States. Among them are the Psilophyton, which disappeared with the Devonian, and the ambiguous characters of which approached at the same time the ferns by the Hymenophyles, the Lycopodiaceæ by Psilotum, and the Rhizocarpes by Pilularia.