Page:EB1911 - Volume 20.djvu/597

MESOZOIC] Sphenozamites, chiefly from French localities, which are referred to the Cycads because of their similarity to the pinnate fronds of modern Cycadaceae. In the succeeding Triassic system Cycadean plants become much more abundant, especially in the Keuper period; from Rhaetic rocks a still greater number of types have been recorded, among which may be mentioned Nilssonia (fig. 10), Anomozamites, Pterophyllum, Otozamites, Cycadites (fig. 11). The species of Nilssonia shown in fig. 10 (N. compta) is a characteristic member of the Jurassic flora, practically identical with a form from Rhaetic rocks described as Nilssonia polymorpha. The large frond of Cycadites represented in fig. 11 (C. Saportae) is from the Wealden strata of Sussex, and possibly identical with Cycadites tenuisectus from Portugal. In addition to these genera there are others, such as Ctenozamites, Ctenis, and Podozamites, the position of which is less certain. Ctenozamites occurs chiefly in the Rhaetic coal-bearing beds of Scania, and has been found also in the Liassic clays of Dorsetshire and in the Inferior Oolite beds of Yorkshire, as well as in Rhaetic strata in Persia and elsewhere; it is characterized by its bipinnate fronds, and may be compared with the recent Australian genus Bowenia—peculiar among living Cycads in having bipinnate fronds. Ctenis has been incorrectly placed among the ferns by some authors, on account of the occurrence of supposed sporangia on its pinnae; but there is reason to believe that these so-called sporangia are probably nothing more than prominent papillose cells of the epidermis. Podozamites (fig. 12) is usually considered to be a Cycad, but the broad pinnae (or leaves) and their arrangement on the axis suggests a possible relationship with the southern coniferous genus Agathis, represented by the Kauri pine and other recent species. The considerable variation in the size of the pinnae of Podozamites, as represented by species from the Jurassic rocks in the Arctic regions and various European localities, recalls the variation in length and breadth of the leaves of Agathis. With regard to the distinguishing features and the distribution of the numerous Cycadean leaves of Mesozoic age, the most striking fact is the abundance of

fronds, which there is good reason to refer to the Cycadales in Upper Triassic, Rhaetic, Jurassic and Wealden rocks in India, Australia, Japan, China and elsewhere in the southern hemisphere, as well in North America, Greenland, and other Arctic lands and throughout Europe. It is noteworthy that Tertiary plant-beds have yielded hardly any specimens that can be recognized as Cycads.

A more important question is, What knowledge have we of the reproductive organs and stems of these fossil Cycads? Cycadean stems have recently been found in great abundance in Jurassic and possibly higher strata in Wyoming, South Dakota and other parts of the United States. Cycadean stems have been found also in the uppermost Jurassic, Wealden and Lower Cretaceous rocks of England, India and other parts of the world. An example of an Indian Cycadean stem from Upper Gondwana rocks is represented in fig. 13; the surface of the trunk is covered with persistent bases (fig. 13, A) of the fronds known as Ptilophyllum cutchense, which are practically the same as the European species Williamsonia pecten (fig. 17). In a section of the stem (fig. 13, B) a large pith is seen to occupy the axial region, and this is surrounded by a zone of secondary wood, which appears to differ from the characteristic wood of modern Cycads (see ) in having a more compact structure. It is interesting to find that G. R. Wieland of