Fig. 288. Hausmannia dichotoma. (Specimens from the late Dr Marcus Gunn’s Collection of Upper Jurassic plants, Sutherlandshire; very slightly reduced.)
Hausmannia dichotoma, Dunker[973]. Fig. 288, A, B.
This Wealden species, represented in the North German flora and in beds of approximately the same age at Quedlinburg, has been discovered by Dr Marcus Gunn in Upper Jurassic rocks on the north-east coast of Scotland. The lamina (12 cm. or more in length) is divided into five to seven linear segments and bears a close superficial resemblance to leaves of Baiera and to recent species of Schizaea ([fig. 222], p. 287). Each segment contains one or two main ribs ([fig. 288], A). A similar form is described by Bartholin[974] and by Moeller[975] as H. Forchammeri from Jurassic rocks of Bornholm.
Hausmannia Kohlmanni, Richt. Fig. 278, F.
In this species, instituted by Richter from material obtained from the Lower Cretaceous beds of Strohberg[976], the comparatively slender rhizome bears fronds with petioles reaching a length in extreme cases of 25 cm. but usually of about 10 cm. The lamina (1–7 cm. long and 1–10 cm. broad) is described as leathery, obcordate, and divided into two symmetrical halves by a median sinus which, though occasionally extending more than half-way through the lamina, is usually shallow. The venation consists of two main branches which diverge from the summit of the petiole ([fig. 278], F) and subdivide into dichotomously branched ribs; finer veins (not shown in the drawing) are given off from these at right angles and form more or less rectangular meshes as in other members of the Dipteridinae and in such recent ferns as Polypodium quercifolium ([fig. 231], D, p. 297).
The imperfect lamina represented in [fig. 289] may belong to Hausmannia Richteri or may be a distinct species; it shows some of the finer veins connecting the shorter forked ribs, which formed part of the reticulate ramifying system in the mesophyll. This specimen was obtained from the plant-beds of Culgower on the Sutherlandshire coast, which have been placed by some geologists in the Kimmeridgian series.
The smaller type represented in [fig. 278], E, is referred by Richter to a distinct species, Hausmannia Sewardi[977], founded on a few specimens from the Lower Cretaceous strata of Strohberg. This species is characterised by a stouter rhizome bearing smaller leaves consisting of a short petiole (3–4 cm. long) and an obovate lamina (1–2 cm. long and broad). There are usually two opposite leaflets on each leaf-stalk, and these may be equivalent to the two halves of a single deeply dissected lamina.
Fig. 289. Hausmannia sp. Upper Jurassic, near Helmsdale, Scotland. From a specimen in the British Museum. (Nat. size.)