_Medullosa_ has been mentioned already (see p. 72) because of the interesting and unusually complex type of its vascular anatomy. Each individual stele of the group of three in the stem, however, is essentially similar to the stele of a Heterangium. Though the whole arrangement appears to differ so widely from other stems in the plant world, careful comparison with young stages of recent Cycads has indicated a possible remote connection with that group, while in the primary arrangements of the protosteles a likeness may be traced to the ferns. The roots, even in their primary tissues, were like those of Gymnosperms, but the foliage with its compound leaves was quite fern-like externally. A small part of a leaf is shown in fig. 83, and is clearly like a fern in superficial appearance. The leaves were large, and the leaf bases strong and well supplied with very numerous branching vascular bundles. [Illustration: Fig. 82.—Steles of _Medullosa_ in Cross Section of the Stem A, Primary solid wood; S, surrounding secondary wood.] [Illustration: Fig. 83.—Part of a Leaf of _Medullosa_, known as _Alethopteris_, for long supposed to be a Fern] 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). [Illustration: 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.] [Illustration: 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.