In one of the earliest English books on fossil plants, the Antediluvian Phytology by Artis[284], a specimen from the Carboniferous sandstone of Yorkshire is figured as Aphyllum cristatum, and a similar fossil is described as A. asperum. These are impressions of Lepidodendron stems in which the characteristic leaf-cushions are replaced by smooth and slightly convex areas with a narrow central ridge. To this type of specimen Presl gave the name Aspidiaria[285], under the impression, shared by subsequent writers, that the supposed external features were entitled to generic recognition.
It is to Stur[286] that we owe the first satisfactory interpretation of fossils included under the name Aspidiaria: he showed that on the removal of the projecting convex areas from some of his specimens a typical Lepidodendron leaf-cushion was exposed ([fig. 144], A, a). The Aspidiaria condition ([fig. 144], A, b) represents the inner face of the detached shell of outer bark of a Lepidodendron stem, while in the Bergeria casts we have a view of the external face of a stem deprived of its superficial tissues.
In a Lepidodendron stem embedded in sediment the more delicate portions of the leaf-cushions would tend to shrink away from the internal and more resistant tissues of the outer cortex, thus producing spaces between each cushion; further decay would cause rupture of the leaf-traces and the superficial tissues would thus be separated from the rest of the stem. The tendency of Lepidodendron stems to split along the line of phellogen in the outer cortex is seen in [fig. 148], A, g. The deposition of sediment on the exposed inner face of this cortical shell would result in the production of a specimen of the Aspidiaria type: the reticulum enclosing the spirally disposed convex areas is formed by the impression of the firmer tissue between the leaf-cushions.
vi. Lepidodendroid axes known as Ulodendron and Halonia.
a. Ulodendron.
This generic name was suggested by Lindley and Hutton[287] for two specimens from the English Coal-measures characterised by leaf-cushions like those of a Lepidodendron, but distinguished by the presence of two vertical rows of large and more or less circular cup-shaped scars. These authors, while recognising the possibility that the fossils might be identical with Lepidodendron, regarded them as generically distinct. The generic title Ulodendron, though no longer denoting generic rank, is still applied to certain shoots of lycopodiaceous plants which may belong to the genera Lepidodendron, Bothrodendron, and according to some authors[288], also to Sigillaria.
The large specimen from the Belgian coal-measures, represented in [fig. 211], affords a good example of the Ulodendron form of shoot of the genus Bothrodendron, which is described on [page 249]. The specimen shown in [fig. 157] shows the Ulodendron shoot of Lepidodendron Veltheimianum.
Casts of large Ulodendron scars are occasionally met with as separate fossils bearing a resemblance to an oval shell.
In Steinhauer’s paper on Fossil Reliquiae[289] a drawing is given of a Ulodendron stem under the name Phytolithus parmatus and a similar stem specifically identical with that shown in [fig. 157] was figured by Rhode[290], one of the earliest writers on fossil plants, under the comprehensive designation “Schuppenpflanze.”
