the greatest errors may arise, as they have in the past. For example, the systematic position of Elephas primigenius having been clearly established, the inference was thought legitimate that, as the modern representatives of the genus were confined to tropical or subtropical countries, the boreal regions must have enjoyed a similar climate when they were inhabited by these ancient elephants. It was, however, discovered that their skin was clothed with wool and long hair, and that, consequently, they were adapted to endure a cold climate. In plants, the structure of the fruit would in most cases teach nothing as to the temperature and humidity of the atmosphere in which, or the kind of soil upon which, the organism grew, though it would be of the first importance in determining systematic position. On the other hand, the root, the leaves, and the tissues of the plant, would be of only secondary importance in regard to systematic position, but of the highest value in determining physical condition. In regard to Lepidodendron, its singular roots would seem to imply that it derived a large amount of moisture through them from a moist soil, and so far differed from most living cryptogamia, which obtain it mostly from the atmosphere. The roots of this genus presented in their crowded and long rootlets an immense surface for the absorption of moisture; and in their great abundance of lax cellular tissue possessed the means of containing this moisture, and transmitting it to the foliage.

The leaves of Lepidodendron were simple, lanceolate, acute, and sessile. They had a single medial nerve. The younger branches were densely covered with leaves; and the scars left on the trunk after they perished, give the numerous beautiful markings by which the species have been distinguished. The leaves when found separated from the branches, are called Lepidophylla.

The fruit was a strobilus (t. 56, f. 3), formed from a shortened branch, the leaves of which are converted into scales, that support on their upper surface a single large sporangium (Lepidostrobus, t. 56, f. 4), or perhaps several small ones (Flemingites, t. 56, f. 6). There appear to be both macrospores (t. 56, f. 5) and microspores in the same sporangium. I have examined at length the structure and affinities of these fruits, in a paper published in the Geological Magazine,' vol. ii. p. 433, to which I must refer, without here dwelling further on the subject. Flemingites, although the sporangia are enormously abundant in some coals, have not yet been found connected with any

6

fossil; but specimens of Lepidostrobus, attached to branches of Lepidodendron, have been described by Dr. Paterson, Brongniart, and others; and I have noticed a fine specimen in the Museum of the Edinburgh Botanic Gardens, and others exist in the collections at the British Museum and elsewhere.

In tracing the affinities of Lepidodendron, we have the safest guide in the organs of fructification, and fortunately these have been satisfactorily determined. The sporiferous strobilus shows that it is a true cryptogam; and in general appearance and arrangement of parts, the strobilus can scarcely be distinguished from that of some living Lycopodia, except in the great difference of size; this affinity is strengthened by the character of the leaves, and the structure of the stem. But the possession of both kinds of spores in the same sporangium exhibits stronger affinity to Rhizocarpea than to Lycopodiaceæ.

The structure of the arboreal stem of Lepidodendron is much more complex than that of any known cryptogam. The central axis of irregularly-arranged vascular tissue in Lycopodium is suited to the low stature of the plants of that genus; but in the giant Lepidodendron there is a complexity, which approaches the structure of some dicotyledonous stems. The general arrangement of the tissues, resembling what exists in some Cycadea and Cactaceae, has caused this fossil plant to be referred sometimes to the one, and sometimes to the other of these Orders; but the resemblance is only one of analogy, and not of affinity. The presence of scalariform tissue, of which the woody portion is entirely composed, and the absence of medullary rays, would, even if the fruit were unknown, be sufficient to establish the cryptogamic nature of the plant. A comparison with the Cycadean stem may help us, by the resemblances and differences which will appear, better to understand the stem of Lepidodendron. The Cycads have all a large medulla, composed of large-sized parenchyma; in some genera traversed by numerous vascular bundles, as in Encephalartus, and in others entirely cellular, as in Cycas and Zamia. This is surrounded by a single woody cylinder, or several, everywhere penetrated with medullary rays. Beyond this there is a considerable thickness of parenchyma, composed, in their inner portion, of cells whose length exceeds only slightly their breadth; these gradually lengthen, until they assume an appearance very like the external portion of Lepidodendron. This cortical parenchyma is traversed by the vascular

bundles which supply the leaves. The two stems are evidently built upon the same plan; and were we to substitute scalariform tissue for the gymnospermatous woody tissue, and scalariform utricles for true medullary parenchyma, and finally exclude the medullary rays, the description of the Cycadean stem would apply to that of Lepidodendron. And it deserves special notice, that the surface of the Cycadean trunk is composed of the bases of the old leaves, together with the scales which in some species are interspersed among them, or alternate with them. The leaves do not disarticulate at the circumference of the stem, but at some distance from it, leaving a small portion of the base persistent. The scars of the outer surface of the stem give a different impression from those presented when the persistent bases of the leafstalks are removed. Whoever is even a little familiar with coal fossils is aware that there are two sets of scars on the stems of Lepidodendron -one superficial, the other internal. The fossils that present the first set are generally said to be "corticated " stems, and those exhibiting the others "decorticated." The "bark" is generally converted into a compact structureless coal, the outer surface of which has the one set of scars, and the inner surface the other. I believe this coal is produced by the external of the two epidermal series, and that the outer scars were truly superficial, while the inner were produced by the vessels which passed to the bases of the leaves. The two sets of scars in Cycadean stems are analogous structures; but in Lepidodendron, the layer which bears the scars on its two surfaces is a compact cylinder; while in the Cycadea there is no connecting tissue uniting the bases of the leaves; they are closely packed together, but quite free from each other. It is evident that in many respects the fossil stem had a striking analogy in the arrangement of its parts to that of the recent Cycads, while it was, however, a true Cryptogam; and if we now examine the slender stem of Lycopodium we shall find, I believe, that Lepidodendron, though more highly developed, does not differ essentially from it.

Spring, in his Monographie des Lycopodiacées' (p. 293), describes the stem of this Order as composed of five parts:-1st. The woody axis; 2nd. A layer of delicate cells; 3rd. The liber; 4th. The herbaceous envelope; and, 5th. The epidermis.

The axis is composed of bundles of scalariform vessels, scattered through a very delicate cellular tissue, in a regular figure, which varies

in the different species. This axis is surrounded by a layer of lax, delicate, cellular tissue, which Spring considers to be the channel through which the sap circulates, and the seat of growth in the stem, -the inner portion being developed into wood vessels, and the outer into "liber." The "liber" is composed of elongated cells, with thickened walls. Spring gives to it this name because of its analogy to the liber in dicotyledons. This layer is often so thin that it is difficult to detect. It is surrounded by a thick greenish layer, composed of large elongated cells, with thin walls; and this is covered with an epidermis, consisting of small cells with thick walls. The vascular bundles pass through the various layers of cellular tissue from the axis to the circumference.

The great difference between the stem of Lepidodendron and Lycopodium is the existence of a pseudo-medulla, and the arrangement of the vascular tissue as a solid cylinder in the fossil genus, compared with the central position and loose structure of the vascular tissue in the recent plant. In both the recent and fossil stems, the vascular tissues are surrounded by a zone of thin-walled cells, which has disappeared in all the dried specimens of Lycopodium I have examined, leaving the axis free, and which, as we have seen, is very rarely preserved in Lepidodendron.

Calamites.-Few fossils have been more misunderstood than the set of plants to which the name Calamites is given. One of the least errors regarding them was that which placed the stem upside down, and made the cylindrical roots its leaves. Calamites is rarely preserved so as to exhibit structure, being almost always converted into amorphous coal, and exhibiting an apparently furrowed and jointed stem, somewhat resembling the recent Equisetacea. The few specimens that have been found with the internal organization of the stems preserved, show a structure different from what had been assumed to be that of Calamites, and have been constituted into the genus Calamodendron. Like Lepidodendron, Calamites must have been a very brittle plant, as its remains occur in such a fragmentary condition, that great difficulty has been experienced in determining the different parts of the plant. The branchlets and foliage have been referred to the genus Asterophyllites, supposed to be independent aquatic plants, and the fruits form the genus Volkmannia.

The stem of Calamites was formed on a different plan from that of

Lepidodendron. Mr. Binney is at present engaged in preparing an account of its internal structure, with copious illustrations, which will be more complete than any hitherto published, because of the abundance of well-preserved specimens contained in his cabinet, the result of so many years' devotion to the study of the fossils of the coal measures. I shall therefore content myself with a hasty sketch of the genus. The specimens hitherto figured by Petzholdt, Corda, Goppert, Sternberg, Unger, and others, have generally wanted the cellular tissue of the axis and of the epidermis. The specimens which Mr. Binney has shown me exhibit, as I believe, the whole structure from the centre to the circumference. The axis (t. 55) consists of a considerable mass of cellular tissue without any vascular bundles penetrating it. This is surrounded by a solid cylinder of wood, formed entirely of scalariform vessels, and without (in all the specimens I have examined) any trace of medullary rays. The vascular tissue was developed from a series of equidistant points near the circumference of the cellular tissue, and grew outwards and laterally until they united in a continuous cylinder, fluted on the inner surface, and with the flutings filled with the cellular tissue of the axis. The early vascular bundles in the young stems of exogenous plants have a similar origin, but they speedily unite to form a woody cylinder, with a clearly defined and smooth inner surface towards the pith. This early condition is permanent in the stem of some arborescent species of Cactus, which, in this respect, closely resembles Calamites; but it is only a similarity in the arrangement of the parts, without any true affinity, for the stems differ as much as Lepidodendron does from Cycas. The woody cylinder formed constrictions at regular intervals round the cellular axis, as in some recent Artocarpeæ. Beyond the woody cylinder there was a thin epidermal layer of parenchyma, which is less seldom preserved than even that of the interior.

The flutings and constrictions of the stem described as external were on the interior of the woody cylinder. The parenchyma having generally disappeared in fossilization, the wood alone formed the thin layer of coal that is generally all that remains to indicate the existence of the fossil. This is always furrowed longitudinally, and barred at intervals, apparently externally; but the examination of specimens, in which the structure is preserved, show that there was no fluting on the outer surface. Richter and Unger, in their Palæontologie des