The connection between this plant and certain large three-ribbed seeds known as Trigonocarpus is strongly suspected, though actual continuity is not yet established in any of the specimens hitherto discovered. These seeds have been mentioned before ([p. 76] and [p. 82]). They were larger than the other fossil seeds which we have mentioned, and, with their fleshy coat, were similar in general organization to the Cycads, though the fact that the seed coat stood free from the inner tissues right down to the base seems to mark them as being more primitive (cf. [fig. 55], [p. 76]).
Of impressions of the Pteridosperms the most striking is, perhaps, the foliage known as Neuropteris (see [fig. 6], [p. 13]), to which the large seeds are found actually attached (cf. [fig. 85]).
Fig. 84.—Diagrammatic Section of a Transverse Section of a Seed of Trigonocarpus
S, Stone of coat with three main ridges and six minor ones. F, Flesh of coat: i f, inner flesh; n, nucellus, crushed and free from coat; s, spore wall.
Fig. 85.—Fragment of Foliage of Neuropteris with Seed attached, showing the manner in which the seeds grew on the normal foliage leaves in the Pteridosperms
Ever-increasing numbers of the “ferns” are being recognized as belonging to the Pteridosperms, but Heterangium, Lyginodendron, and Medullosa form the three principal genera, and are in themselves a series indicating the connection between the fernlike and Cycadean characters.
Before the fructifications were suspected of being seeds the anatomy of these plants was known, and their nature was partly recognized from it alone, though at that time they were supposed to have only fernlike spores.
The very numerous impressions of their fernlike foliage from the Palæozoic rocks indicate that the plants which bore such leaves must have existed at that time in great quantity. They must have been, in fact, one of the dominant types of the vegetation of the period. The recent discovery that so large a proportion of them were not ferns, but were seed-bearing plants, alters the long-established belief that the ferns reached their high-water mark of prosperity in the Coal Measure period. Indeed, the fossils of this age which remain undoubtedly true ferns are far from numerous. It is the seed-bearing Pteridosperms which had their day in Palæozoic times. Whether they led directly on to the Cycads is as yet uncertain, the probability being rather that they and the Cycads sprang from a common stock which had in some measure the tendencies of both groups.