The parichnos scars are shown on the leaf-scar and cushion in fig. 146, C. In the lower leaf-cushion shown in fig. 146, E, the infra-foliar parichnos scars, _p_, are clearly seen, but the preservation of the leaf-scar is not sufficiently good to show them on that part of the fossil. In the upper cushion (fig. 146, E) the position of the parichnos arms is shown on the leaf-scar, but the infra-foliar parichnos scars are hidden by two small spiral shells. The genus _Spirorbis_, to which these shells are referred, appears to have persisted from the Silurian epoch to the present day. The comparatively frequent occurrence of _Spirorbis_ shells on the leaves and other parts of Palaeozoic plants, has recently been dealt with in a paper by Barrois[245] who discusses in detail the habitats of these small animals from the point of view of the conditions under which the plants were preserved. In a note by Malaquin appended to Barrois’ paper the belief is expressed that _Spirorbis_ lived on pieces of Palaeozoic plants which lay under water. The fact that with one exception all the Spirorbis shells on the specimen of _Lepidodendron_, of which two leaf-cushions are shown in fig. 146, E, occur on the large parichnos scars on the cheeks of the cushions, suggests the possibility that the escape of gases from the parichnos tissue may have rendered the position attractive to the _Spirorbis_. It can hardly be accidental that the shells occur on the parichnos strands. This fact recalls the view held by Binney[246] and accepted with favour by Darwin[247] that _Lepidodendron_ and other coal-forest trees may have lived with the lower parts of the stems in sea water. Above the leaf-scar is a fairly deep triangular or crescentic pit (fig. 146, C, _l_) known as the ligular pit from the occurrence on younger shoots of a delicate organ like the ligule of _Isoetes_ (fig. 132) embedded in a depression in the upper part of the leaf-cushion. The ligule was first figured in _Lepidodendron_ by Solms-Laubach[248] and described in English material by Williamson under the name of the adenoid organ[249]. In some Lepidodendron stems a second triangular depression may occur above the ligular pit, the meaning of which is not clear: this has been called the triangulum by Potonié[250]. Stur[251] suggested that it may represent the position occupied by a sporangium in Lepidodendron cones. It is important to remember that as a branch increases in girth the leaf-cushions are capable of only a certain amount of growth: when the limit is reached they are stretched farther apart and thus the narrow groove which separates them is converted in older stems into a comparatively broad and flat channel, thus altering the surface characters. [Illustration: FIG. 146. Lepidophloios and Lepidodendron leaf-cushions.