The species to which Potonié has applied the generic name Alloiopteris[1594] also illustrates the impossibility of drawing a sharp line between Pecopteris and Sphenopteris. The fronds already described in chapter XXV. under the designation Corynepteris bear pinnules with a contracted base; in some species the lamina is lobed, but in others ([fig. 354], G) it is entire with a midrib nearer one edge than the other. The species which Potonié assigns to Alloiopteris, like many other Sphenopteroid and Pecopteroid fronds, are characterised by the occurrence of an abnormal pinnule (aphlebia) at the base of each pinna ([fig. 354], G, p. 535). Young fronds of Pecopteris are occasionally met with showing very clearly the circinate vernation of the pinnae as in the leaves of Cycas and Angiopteris represented in [fig. 220], p. 283. The genus Spiropteris was created by Schimper[1595] for coiled unexpanded fronds of fossil ferns; it is however superfluous to apply a distinctive term to specimens of this kind.
The designation Pecopteris is employed chiefly for leaves of Palaeozoic age which are unknown in the fertile state, or do not afford sufficient evidence as to the nature of the sporangia to justify the substitution of a special generic name. Many Mesozoic species have also been referred to Pecopteris, but most of these are more appropriately included in Brongniart’s later genus Cladophlebis. The pinnules of Cladophlebis, as Brongniart pointed out, are intermediate between Pecopteris and Neuropteris; they are usually attached by the whole breadth of the base, as in Pecopteris, but the more acute origin, more arched form, and more frequent dichotomy of the lateral veins are features shared by Neuropteris. As a rule, Mesozoic sterile fronds with straight or folded, entire or dentate pinnules are of the Cladophlebis type: this genus is especially characteristic of Rhaetic and Jurassic floras. Examples of Cladophlebis pinnules are shown in figs. [256], [257] (pp. 340, 342). It is to be regretted that authors do not make more use of the generic name Cladophlebis in describing sterile fronds, instead of following the misleading and unscientific practice of employing such genera as Pteris, Asplenites, and others on wholly insufficient grounds.
LIST OF WORKS REFERRED TO IN THE TEXT.
Amalitzky, W. (01) Sur la découverte, dans les dépôts permiens supérieurs du Nord de la Russie, d’une flore Glossoptérienne &c. Compt. Rend. vol. CXXXII. p. 591.
Arber, E. A. N. (02) On the distribution of the Glossopteris Flora. Geol. Mag. vol. IX. p. 346.
—— (022) The Clarke collection of fossil plants from New South Wales. Quart. Journ. Geol. Soc. vol. LVIII. p. 1.
—— (03) Notes on some fossil plants collected by Mr Molyneux in Rhodesia. Quart. Journ. Geol. Soc. vol. LIX. p. 288.
—— (05) Catalogue of the fossil plants of the Glossopteris Flora in the British Museum. A monograph of the Permo-Carboniferous Flora of India and the Southern Hemisphere. London.
—— (052) The Sporangium-like organs of Glossopteris Browniana, Brongn. Quart. Journ. Geol. Soc. vol. LXI. p. 324