Dipteridinae.

Dictyophyllum.

This genus was founded by Lindley and Hutton for a pinnatifid leaf from the Jurassic rocks of Yorkshire which they regarded as probably dicotyledonous and named D. rugosum[946]. Several ferns of this genus have since been found with well-preserved sori which demonstrate a close similarity to the recent fern Dipteris. Dictyophyllum may be defined as follows:—

Fronds large and palmate, characterised by the equal dichotomy of the main rachis into two arms which curve outwards and then bend inwards ([fig. 281]); from the surface of each arm are given off numerous spreading pinnae with a lamina more or less deeply dissected into lobes varying in breadth and in the form of the apex. Each lobe has a median vein, from which branches are given off approximately at right angles and then subdivide into a reticulum, in the meshes of which the veinlets end blindly ([fig. 282], A and E). Sori composed of annulate sporangia are crowded on the lower surface of the lamina. In habit and in sporangial characters the genus closely resembles Dipteris, and in the branching of the frond suggests comparison with Matonia. The rhizome (Rhizomopteris) is creeping and dichotomously branched, bearing leaf-scars with a horse-shoe form of vascular strand.

Fig. 281. Dictyophyllum exile. (After Nathorst; much reduced.)

Dictyophyllum is represented by several types to which various specific names have been assigned, the distinguishing features being the form of the pinna lobes, the degree of concrescence between the basal portions of the pinnae, and similar features which in some cases can only be safely used as criteria when large specimens are available for comparison.

Dictyophyllum exile (Brauns). Figs. [281], [282], D, E.
1862.Camptopteris exilis, Brauns, Palaeontograph. IX. p. 54.
1867.Dictyophyllum acutilobum, Schenk, Foss. Flor. Grenz. p. 77, Pls. XIX. XX.
1878.D. exile, Nathorst, Flora vid Bjuf, I. p. 39, Pl. V. fig. 7.
—D. acutilobum, ibid. Pl. XI. fig. 1.

The restoration, after Nathorst[947], shown in [fig. 281] illustrates the habit of this striking fern, examples of which or of closely allied species are recorded from Rhaetic rocks of Germany, Scania, Persia, Bornholm, Tonkin, China, and elsewhere[948]. The petiole, reaching a length of 60 cm., forks at the apex into two equal arms leaving between them an oval space and occasionally crossing one another. The axes of these branches are twisted so that the pinnae, which may be as many as 24 on each arm, and arise from the inner side, by torsion of the axes assume an external position. An interesting analogy as regards the twisted rachis of Dictyophyllum exile and Camptopteris is afforded by the leaves of the Cycads, Macrozamia Fawcettiae and M. corallipes, which are also characterised by the torsion of the rachis. The habit, justly compared by Nathorst with that of Matonia pectinata, affords another illustration of the common occurrence in older ferns of a dichotomous system of branching. The pinnae, characterised by circinate vernation, reach a length of 60 cm. and are divided into linear lobes inclined obliquely or at right angles to the pinna axis. The whole of the under surface of the lamina may be covered with sporangia, 4–7 sporangia in each sorus; the annulus is incomplete and approximately vertical ([fig. 282], D). The rhizome is probably represented by the dichotomously branched axis described by Nathorst from Scania as Rhizomopteris major; the leaf-scars show a horse-shoe leaf-trace.

Fig. 282.

(After Nathorst; A, B, C, E, ⅔ nat. size.)

Dictyophyllum Nathorsti Zeiller[949].

This type, represented by a splendid series of specimens from the Rhaetic beds of Tonkin, agrees very closely with D. exile. It differs, however, in the basal parts of the pinnae which are concrescent for a length of 5 to 8 cm. instead of free as in D. exile; and, to a slight degree, in the form of the ultimate segments. In habit and in soral characters the two species are practically identical. Each sorus contains 5 to 8 sporangia, which are rather larger than those of Dipteris.

Dictyophyllum rugosum, Lind. and Hutt. Fig. 283.
1828.Phlebopteris Phillipsii, Brongniart, Hist. vég. foss. p. 377, Pl. CXXXII. fig. 3; Pl. CXXXIII. fig. 1.
1829.Phyllites nervulosis, Phillips, Geol. Yorks. p. 148, Pl. VIII. fig. 9.
1834.Dictyophyllum rugosum, Lindley and Hutton, Foss. Flor. II. Pl. CIV.
1836.Polypodites heracleifolius, Goeppert, Foss. Farn. p. 344.
1849.Camptopteris Phillipsii, Brongniart, Tableau, p. 105.
1880.Clathropteris whitbyensis, Nathorst, Berättelse, p. 83.

This species, which is characteristic of Jurassic rocks, is less completely known than the two types described above, but in the form and venation of the pinnae there is little difference between the Rhaetic and Jurassic plants. The leaves of the Jurassic species appear to have been smaller and more like those of Dipteris conjugata ([fig. 231]); there are no indications of the existence of the two curved arms at the summit of the petiole which form so striking a feature in D. exile and D. Nathorsti. No sporangia have been found on English specimens, but it is safe to assume their agreement with those of other species. A more complete list of records of D. rugosum is given in the first volume of the British Museum Catalogue of Jurassic plants[950].

Fig. 283. Dictyophyllum rugosum (Lind. and Hutt.). (Brit. Mus. Nat. size.)

Nathorst[951] has recently drawn attention to certain differences between Dictyophyllum and Dipteris. The pinnate division of the pinnae is not represented in the fronds of the recent species, but this method of lobing, which is a marked characteristic of Dictyophyllum, is less prominent in Clathropteris; and in Camptopteris lunzensis Stur[952], an Austrian Upper Triassic species, the pinnae are entire. In Dictyophyllum the sori cover the whole lower surface of the leaf; in Dipteris they are more widely separated and the sporangia have a diameter of 0·02 mm., but in Dictyophyllum the diameter is 0·4–0·6 mm. Moreover in Dictyophyllum the sori contain 5 to 8 sporangia, whereas in Dipteris they are much more numerous. Despite these differences it is clear, as Nathorst says, that Dictyophyllum, Clathropteris, and Camptopteris are existing types very closely allied to Dipteris. It is a matter of secondary importance whether we include all in the Dipteridinae or follow Nathorst’s suggestion and refer the fossil genera to the separate family Camptopteridinae.

Thaumatopteris.

This genus, founded by Goeppert[953] for a Rhaetic plant from Bayreuth, is by some authors[954] regarded as identical with Dictyophyllum, but it has recently been resuscitated by Nathorst[955] for specimens which he names T. Schenki, formerly included by Schenk in his species T. Brauniana[956]. It bears a close resemblance, in the long linear pinnules with an entire or crenulate margin, to Dictyophyllum Fuchsi described by Zeiller[957] from Tonkin, and it would seem hardly necessary to adopt a distinctive generic designation. The sporangia have a vertical or slightly oblique annulus and the rhizome is similar to that of Dictyophyllum exile. The habit of the genus is shown in [fig. 284], which represents one of the German Rhaetic species.

Fig. 284. Thaumatopteris Münsteri. (From a specimen in the Bergakademie, Berlin; ⅓ nat. size.)

Clathropteris.
Clathropteris meniscoides, Brongn. Fig. 285.

Clathropteris, founded by Brongniart[958] for Rhaetic specimens from Scania, agrees very closely with some species of Dictyophyllum, but in view of the more rectangular form of the venation-meshes it is convenient to retain both names. The type-species was originally named Filicites meniscoides[959] and afterwards transferred to Clathropteris. An examination of Brongniart’s specimens has convinced Nathorst of the specific identity of C. meniscoides and C. platyphylla. The Tonkin leaves described by Zeiller[960] under the latter name should, therefore, be included in C. meniscoides, which may be thus defined:

The petiolate frond is characterised by an equal dichotomy of the rachis, as in Dictyophyllum; each branch bore 5–15 pinnae, disposed en éventail, reaching a length of 20–30 cm. and fused basally as in D. Nathorsti Zeill. Pinnae linear lanceolate, slightly contracted at the lower end and gradually tapered distally. The lamina, 3–14 cm. broad, is characterised by obtusely pointed marginal lobes. From the midrib of each pinna lateral veins are given off at a wide angle, and adjacent veins are connected by a series of branches which divide the lamina into a regular reticulum of rectangular and polygonal meshes ([fig. 285]). The sori are abundant and contain 5–12 sporangia like those of Dictyophyllum.

Fig. 285. Clathropteris meniscoides. From Rhaetic rocks near Erlangen. [M.S.]

Fig. 286. Clathropteris egyptiaca. (Nat. size.) a, b, pieces of main ribs in grooves.

What is probably the rhizome of this species has been described by Nathorst (Rhizomopteris cruciata); it is similar to that of Dictyophyllum, but the leaf-scars are more widely separated. This species occurs in Upper Triassic, Rhaetic or Lower Jurassic rocks of Scania, France, Germany, Switzerland, Bornholm, North America, China, Tonkin, and Persia and is represented by fragments in the Rhaetic beds of Bristol[961].

Clathropteris egyptiaca Sew.[962] Fig. 286.

The specimen on which this species was founded was discovered in the Nubian Sandstone east of Edfu; the age of the beds is uncertain, but the presence of Clathropteris suggests a Lower Jurassic or Rhaetic horizon[963]. Seven strong ribs radiate through the lamina from the summit of the petiole; at a and b small pieces of the projecting ribs are shown in the grooves. From the main veins slender branches are given off at right angles and, as seen in the enlarged drawing, these again subdivide into a delicate reticulum with free-ending veinlets.

Fig. 287. Camptopteris spiralis. (After Nathorst. Much reduced.)

Camptopteris.
Camptopteris spiralis, Nath. Figs. [282], C; [287].

Nathorst proposed this generic name for Rhaetic fronds[964] resembling those of Clathropteris and Dictyophyllum, but differing in the form of the pinnae and in habit. The habit of the type-species, C. spiralis, is shown in [fig. 287]. An examination of the specimens in the Stockholm Museum convinced me of the correctness of Nathorst’s restoration[965]. Each of the forked arms of the rachis bore as many as 150–160 long and narrow pinnae characterised by an anastomosing venation ([fig. 282], C) and by a spiral disposition due to the torsion of the axes. The sporangia agree in essentials with those of Dictyophyllum.

Hausmannia.

A critical and exhaustive account of this genus has been given by Prof. Von Richter[966] based on an examination of specimens found in the Lower Cretaceous rocks of Quedlinburg in Germany. The name was proposed by Dunker[967] for leaves from the Wealden of Germany characterised by a deeply dissected dichotomously branched lamina. Andrae subsequently instituted the genus Protorhipis[968] for suborbicular leaves with dichotomously branched ribs from the Lias of Steierdorf. A similar but smaller type of leaf was afterwards described by Zigno[969] from Jurassic beds of Italy as P. asarifolius, and Nathorst[970] figured a closely allied form from Rhaetic rocks of Sweden. While some authors regarded Hausmannia and Protorhipis as ferns, others compared them with the leaves of Baiera (Ginkgoales); Saporta suggested a dicotyledonous affinity for leaves of the Protorhipis type. The true nature of the fossils was recognised by Zeiller[971], who called attention to the very close resemblance in habit and in soral characters to the recent genus Dipteris. A comparison of the different species of Dipteris, including young leaves ([fig. 231], p. 297), with those of the fossil species reveals a very striking agreement[972]. There can be no doubt, as Richter points out, that the names Hausmannia and Protorhipis stand for one generic type.

Hausmannia may be defined as follows:

Rhizome creeping, slender, dichotomously branched; leaf-stalks slender (2–25 cm. long), bearing a leathery lamina (1–12 cm. long and broad), wedge-shaped below, occasionally cordate or reniform, entire or more or less deeply lobed into broad linear segments. The leaf is characterised by dichotomously branched main ribs which arise from the summit of the rachis as two divergent arms and radiate in a palmate manner, with repeated forking, through the lamina. Lateral veins are given off at a wide angle, and, by subdivision, form a fairly regular network similar to that in Dictyophyllum, Clathropteris, and Dipteris.

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.)

It is interesting to compare these different forms of Hausmannia with the fronds of recent species of Dipteris represented in [fig. 231]. The more deeply dissected type, such as H. dichotoma, closely resembles D. Lobbiana or D. quinquefurcata, while the more or less entire fossil leaves ([fig. 278], E, F and [fig. 289]) are very like the somewhat unusual form of Dipteris conjugata shown in [fig. 231], B, p. 297.

Other species of the genus are recorded from Liassic rocks of Steierdorf[978] (Hungary) and of Bornholm[979]. Nathorst[980] has described a small Rhaetic species from Scania: a French Permian plant described by Zeiller[981] and compared by him with H. dichotoma, may be a Palaeozoic example of this Dipteris-like genus.

Some segments of leaves from the Eocene beds (Middle Bagshot) of Bournemouth, and now in the British Museum, described by Gardner and Ettingshausen[982] as Podoloma polypodioides, bear a close resemblance in the venation to the lamina of Dipteris conjugata.