No other species of Equisetites affords such numerous examples of tubers as this Wealden plant. By some of the earlier writers the detached tubers of E. Burchardti were described as fossil seeds under the name Carpolithus.
Fig. 66. Equisetites Yokoyamae Sew. From specimens in the British Museum. Nat. size.
The specimens shown in fig. 66 have been referred to another species, E. Yokoyamae Sew.[540]; they were obtained from the Wealden beds of Sussex, but according to Mr Rufford, who discovered them, the smaller tubers of this species are not found in association with those of E. Burchardti. The stems are very narrow and the tubers have a characteristic elliptical form; the species is of little value botanically, but it affords another instance of the common occurrence of these tuberous branches in the Wealden Equisetums.
Similar fossil tubers, on a much larger scale, have been found in association with the Triassic Equisetites arenaceus; with E. Parlatori Heer[541], a Tertiary species from Switzerland, and with other Mesozoic and Tertiary stems. E. Burejensis[542], described by Heer from the Jurassic rocks of Siberia, bears a close resemblance to the Wealden species.
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The description of the above species by no means exhausts the material which is available towards a history of fossil Equisetums. The examples which have been selected may serve to illustrate the kind of specimens that are usually met with, as well as some of the possible sources of error which have to be borne in mind in the description of species.
Such Tertiary species as have been recorded need not be considered; they furnish us with no facts of particular interest from a morphological point of view. The wide distribution of Equisetites, especially during the Jurassic period, is one of the most interesting lessons to be learnt from a review of the fossil forms. No doubt a detailed comparison of the several species from different parts of the world would lead us to reduce the number of specific names; and at the same time it would emphasize the apparent identity of fossils which have been described from widely separated latitudes under different names.
Specimens of Equisetites are occasionally found in plant-bearing beds apart from the other members of a Flora; this isolated manner of occurrence suggests that the plant grew in a different station from that occupied by Cycads and other elements of the vegetation[543].
A selection of Triassic and Jurassic species arranged in a tabular form demonstrates the world-wide distribution of this persistent type of plant[544].