Chapter X: I. EQUISETALES (continued) (3)

The breadth of the coal on the same form of cast may vary considerably; on this account, and for various other reasons, such a character can have but little value. Our knowledge of anatomy may often help us to interpret certain features of internal casts and to appreciate apparently unimportant details. One occasionally notices that a Calamite pith-cast has large infranodal canals, and in some specimens each internodal ridge may be traversed by a narrow median line or small groove; large infranodal canal casts suggest the type of stem referred to the subgenus _Arthrodendron_, and the median line on the ridges may be due to bands of hard tissue in each principal medullary ray.

In attempting to identify pith-casts the student must keep in view the probable differences presented by the branching rhizome, the main aerial branches and the finer shoots of the same individual. The long internodal ridges of some casts may be mistaken for the parallel veins of such a leaf as _Cordaites_, a Palaeozoic Gymnosperm, if there are no nodes visible on the specimen. The fossil figured by Lindley and Hutton[755] as _Poacites_, and regarded by them as a Monocotyledon, is no doubt a portion of a Calamite with very long internodes. An interesting example of incorrect determination has recently been pointed out by Nathorst[756] in the case of certain casts from Bear Island, originally described by Heer as examples of _Calamites_; the vertical rows of leaf-trace casts on a _Knorria_ were mistaken for the ribs of a Calamite stem. The specimens in the Stockholm Museum fully bear out Nathorst’s interpretation. The undulating course of internodal ridges and grooves is not in itself a character of specific value. If a Calamite stem were bent slightly, the wood and medullary-ray tissues on the concave side might adapt themselves to the shortening of the stem by becoming more or less folded, and a cast of such a stem would show undulating ridges and grooves on one side and straight ones on the other[757].

A convenient classification of Calamite casts was proposed by Weiss in 1884, founded chiefly on the number and manner of occurrence of branch-scars—or rather branch-depressions—on the surface of pith-casts. Weiss[758] recognised the imperfection of his proposed grouping, and Zeiller[759] has also expressed reasonable doubts as to the scientific value of such group-characters. Weiss instituted three subgenera—_Calamitina_, _Eucalamites_ and _Stylocalamites_, which are made use of as convenient terms in descriptive treatment of Calamite casts. The following account of a few of the more typical casts may serve to illustrate the methods employed in the description of such specimens; the synonomy given for the different species is not intended to be complete, but it is added with a view to drawing attention to the necessity for careful comparison in systematic work.

A. _Calamitina._

From a specimen in the Manchester Museum, Owens College. ¼ nat.
size.]

This sub-genus of _Calamites_, as instituted by Weiss[760], includes Calamitean stems or branches, which are characterised by the periodic occurrence of branch-whorls usually represented by fairly large oval or circular scars just above a nodal line (figs. 99, 100 and 101). The branch-scars may form a row of contiguous discs, or a whorl may consist of a smaller number of branches which are not in contact basally. A form described by Weiss as _C. pauciramis_, Weiss[761], has only one branch in each whorl, as represented by a single large oval scar on some of the nodes of the cast. A stem of this form is by no means a typical _Calamitina_, but it serves to show the existence of forms connecting Weiss’ sub-genera _Calamitina_ and _Eucalamites_. The number of internodes and nodes between the branch whorls varies in different specimens, and is indeed not constant in the same plant. Each nodal line bears numerous elliptical scars which mark the points of attachment of leaves; each branch-whorl is situated immediately above a node, and in some forms this nodal line pursues a somewhat irregular course across the stem, following the outlines of the several branch-scars[762]. The surface of the internodes is either perfectly smooth or it is more frequently traversed by short longitudinal ridges or grooves probably representing fissures in the bark of the living stem; these are indicated by lines in fig. 99 and by elongated elliptical ridges in fig. 101. On young stems the leaves are occasionally found in place, as for example in an example figured by Weiss[763] (_C. Göpperti_), or we may have leaf-verticils still in place in much older and thicker branches[764] (cf. fig. 85, p. 330).

It occasionally happens that the bark of _Calamitina_ stems has been preserved as a detached shell[765] reminding one of the sheets of Birch bark often met with in forests, the separation being no doubt due in the fossil as in the recent trees to the manner of occurrence of the cork-cambium.

In a few cases branches have been preserved still attached to a stem or branch of higher order; examples of such specimens are figured by Lindley and Hutton[766], Stur[767], and others. Grand’Eury[768] has given an idealised drawing of a typical _Calamitina_ bearing a whorl of branches with the foliage and habit of _Asterophyllites equisetiformis_. The specimen on which this drawing is based is in the Natural History Museum, Paris; it shows Asterophyllitean branches in organic connection with a Calamitean stem, but it is not quite clear if the stem is a true _Calamitina_. A large drawing of this interesting specimen is given by Stur[769] in his monograph on _Calamites_, also a smaller sketch by Renault[770] in his _Cours de botanique fossile_. Similar branches of the _Asterophyllites_ type attached to an undoubted _Calamitina_ are figured also by Lindley and Hutton. There is, in short, good evidence that stems of this sub-genus bore branches with Asterophyllitean shoots.

The wood of stems of the Calamitina group of _Calamites_, in some instances at least, was of the _Arthropitys_ type; this has been shown to be the case in some French specimens from the Commentry coal-field[771] and in others described by Stur[772]. The pith-casts of _Calamitina_ are characterised by comparatively short internodes separated by deep nodal constrictions, as shown in fig. 100. From Permian specimens from Neu Paka in Bohemia, described by Stur[773], we learn that there were the usual Calamite diaphragms bridging across the wide pith-cavity at each node. Such a cast as that shown in fig. 100 is often referred to as _Calamites approximatus_ Brongn.; the length of the internodes and the periodic occurrence of branch-scars in the form of circular or oval depressions along a nodal line enable us to recognise the _Calamitina_ casts. Weiss[774] points out that in pith-casts of this form the branch-scars occur on the nodal constriction, and not immediately above the node as is the case on the surface of a typical _Calamitina_. This distinction is however of little or no value; the point of attachment of a branch may be above the nodal line, while on the pith-cast of the same stem the point of origin of the vascular bundles of the branch is on the nodal constriction[775].

The specimen shown in fig. 100 illustrates the appearance of a _Calamitina_ cast. There is a verticil of branch-scars on the lowest nodal constriction; on the right of the pith-cast the broad band of wood is faintly indicated by the smooth surface of the rock (_x_). Other examples demonstrating the existence of a broad woody cylinder in _Calamitina_ stems have been figured by Weiss[776] and other writers, and some good examples may be seen in the British Museum.

_x_, impression of the wood.
(From a specimen in the collection of Mr R. Kidston.)]

We have so far noticed the connection of certain forms of pith-casts (_e.g._ _Calamites approximatus_), and Asterophyllitean shoots with stems of the sub-genus _Calamitina_.

As regards the strobili our knowledge is far from satisfactory. Stur[777] figures some fertile branches bearing long and narrow strobili, either _Palaeostachya_ or _Calamostachys_, in close association with _Calamitina_ stems, and Renault and Zeiller[778] give illustrations of the association of _Calamitina_ stems with large strobili of the _Macrostachya_ form.

Before Weiss proposed the term _Calamitina_, various authors had figured this form of Calamite under a distinct generic name (_e.g._ _Hippurites_ of Lindley and Hutton[779], _Cyclocladia_[780], _Macrostachya_[781], &c.). Stems of this type have also been described by more recent writers under different names, and considerable confusion has been caused by the use of numerous generic designations for forms of _Calamitina_. Some small fragments of _Calamitina_ stems were described by Salter[782] in 1863 as portions of a new species of the Crustacean _Eurypterus_ (_E. mammatus_). In 1869 Grand’Eury proposed the generic name _Calamophyllites_[783] for stems bearing verticils of _Asterophyllites_ shoots; his description of such stems agrees with Weiss’ _Calamitina_, but as Grand’Eury’s name is used in a narrower sense as implying a connection with _Asterophyllites_, it is more convenient to adopt Weiss’ term in spite of the priority of _Calamophyllites_. In the _Fossil flora of Commentry_ we find some flattened stems of the _Calamitina_ type described under different generic names, as _Arthropitys approximatus_[784] and as _Macrostachya_[785].

The determination of distinct species of the sub-genus _Calamitina_ is rendered almost hopeless by the variation in the different branches of the same individual, and by the difficulty of connecting surface-impressions with casts of the pith-cavity.

A typical example of the _Calamitina_ type of _Calamites_ was figured by Sternberg[786] in 1821 as _Calamites varians_. This has been adopted by Weiss[787] as a comprehensive species including several different ‘forms’ of stems, which differ from Sternberg’s fossil in such points as the number of nodes between the branch-whorls and the number of branches in each whorl. The result of this system of nomenclature is the separation of portions of one specific type under different form-names. It must be clearly recognised that accurate specific diagnoses are practically impossible when we have to deal with fragments of plants, some of which are mere pith-casts, while others show the surface features. The specimen represented in fig. 99 agrees with a stem described by Ettingshausen[788] in 1855 as _Calamites Göpperti_, and as a matter of convenience a member of the _Calamitina_ group showing such characters may be referred to as _Calamites_ (_Calamitina_) _Göpperti_ (Ett.). The following list, which includes a few synonyms of this form, may suffice to illustrate the difficulties connected with accurate systematic determinations.

_Calamites_ (_Calamitina_) _Göpperti_ (Ett.). Fig. 99.

1855. _Calamites Göpperti_, Ettingshausen[789].
1869. _Calamites_ (_Calamophyllites_) _Göpperti_, Grand’Eury[790].
1874. _Cyclocladia major_, Feistmantel[791].
1874. _Calamites verticillatus_, Williamson[792].
1876. _Calamitina Göpperti_, Weiss[793].
1884. _Calamites_ (_Calamitina_) _varians abbreviatus_, Weiss[794].
1884. _Calamites_ (_Calamitina_) _varians inconstans_, Weiss[795].
1887. _Calamites Sachsei_, Stur[796].
1888. _Calamophyllites Göpperti_, Zeiller[797].

This species is characterised by the smooth bark, which may be traversed by a few irregular longitudinal fissures; most of the nodes bear a series of small leaf-scars, and at fairly regular intervals a node is immediately succeeded by a circle of contiguous branch-scars, 8–12 in a whorl. The pith-cast of this type of stem has short ribbed internodes separated by rather deep nodal constrictions; the branch-whorls being represented by a series of pits on the nodal constrictions recurring at corresponding intervals to the whorls of branch-scars on the surface of the stem. Leaves narrow and linear in form, like those on Asterophyllitean branches, are occasionally associated with this type of stem.

The fragment of a _Calamitina_ stem shown in fig. 101 is the counterpart of a specimen originally figured by Steinhauer[798] in 1818 as a species of _Phytolithus_. This may be specifically identical with _C. Göpperti_; but it is better to speak of so small a specimen as merely one of the _Calamitina_ stems, to be compared with _Calamites_ (_Calamitina_) _Göpperti_. The specimen measures 14·5 cm. in length and 7 cm. in breadth.

The form of pith-cast represented in fig. 100 is no doubt that of one of the _Calamitina_ species, but as it is seldom possible to determine the connection between such casts and the particular species of stems to which they belong, they are often referred to as _Calamites_ (_Calamitina_) _approximatus_ (Brongn.). The specimen of which fig. 100 is a photograph was originally described and figured by Mr Kidston[799] from the lower Coal-Measures of Ayrshire. Both _Calamites_ (_Calamitina_) _Göpperti_ (Ett.) and _C._ (_Calamitina_) _approximatus_ (Brongn.) are recorded from the Transition, Middle and Lower Coal-Measures[800].

B. _Stylocalamites._

In the members of this sub-genus the branch-scars are either irregular in their occurrence or absent. In some Calamites the branch-scars are very few and far between, and other species appear to have been almost without branches; pith-casts of such stems may be referred to the sub-genus _Stylocalamites_[801].

An exceedingly common Calamitean cast, _C. Suckowi_ Brongn. (fig. 82) affords a good illustration of this type of stem. In the specimen shown in fig. 82 we have a cast of a rhizome, which is rather exceptional in showing three branches in connection with one another. The appearance of the fossil suggests a rhizome, rather than an aerial shoot, bearing lateral branches; the narrowing of the branches and the rapid decrease in the length of the internodes towards the point of attachment being features associated with rhizomes rather than with aerial branches.

_Calamites_ (_Stylocalamites_) _Suckowi_, Brongn. Fig. 82.

1818. _Phytolithus sulcatus_, Steinhauer[802].
1825. _Calamites decoratus_, Artis[803].
1828. _Calamites Suckowi_, Brongniart[804].
1833. _Calamites cannaeformis_, Lindley and Hutton[805].

For more complete lists of synonyms of this species reference should be made to Kidston[806], Zeiller[807], and other authors.

Casts of _Calamites Suckowi_ are characterised by flat or slightly convex internodal ridges separated by shallow depressions, the ridges are rounded at the upper end of each internode, and usually bear circular casts of infranodal canals. There are some unusually large examples of casts of this species in the British Museum from the Radstock Coal-Measures; one of these has a length of 81 cm., and a diameter of 27 cm. Specimens are not infrequently found with verticils of slender roots in close proximity to the nodes of the cast; figures of such root-bearing casts have been given by Lindley and Hutton[808], Weiss[809], and other authors.

Renault[810] has drawn attention to the thinness of the layer of wood which is often associated with large casts of _C. Suckowi_; he concludes that the stems must have possessed little or no secondary wood. In a more recent work by Grand’Eury[811] _Calamites Suckowi_ is spoken of as having had wood of the _Calamodendron_ type, but as wood of this kind has not been found in England, it is suggested that the plant may not have assumed an arborescent habit until late in the Coal-Measure period. During the Lower and Middle Coal-Measures, at which horizon it commonly occurs in England, it may have been herbaceous. This suggestion has little to commend it; the close agreement between _C. Suckowi_ from English and French localities points to a plant of the same form, and we have no satisfactory evidence as to any difference in stem-structure in the two cases.

Stur has figured a specimen of a Calamite cast, which he compares with _C. Suckowi_, surrounded by a band of silicified wood apparently of the _Arthropitys_ type. From this and other facts it would appear probable that some of the English stems with the _Arthropitys_ structure possessed casts referable to _Calamites_ (_Stylocalamites_) _Suckowi_.

We are not in a position to speak with confidence as to the strobili of _C. Suckowi_, but Stur adduces evidence in support of a connection between this species of Calamite and certain Asterophyllitean branches (_Calamocladus equisetiformis_) bearing Calamostachyan cones. He does not appear to have found the foliage-shoots and stems in organic contact, but draws this conclusion from the association of the fertile branches and stems in the same rocks[812]. This species is abundant in the Lower, Middle and Upper Coal-Measures; it has also been recorded from the Millstone Grit[813].

C. _Eucalamites._

In this sub-genus branch-scars occur on every node; the scars never form a contiguous whorl as in _Calamitina_, but there may be from 3 to 10 on each node. The scars of successive nodes often alternate in position, and thus form more or less regular vertical series as shown in fig. 102. The most obvious feature as regards the arrangement of the branch-scars is their spiral disposition on the surface of the pith-cast. The internodes are fairly uniform in length, and there is no periodic recurrence of narrower internodes as in _Calamitina_. From an examination of specimens of _Eucalamites_ in which the pith-cast is covered with a coaly layer representing the carbonised remains of the wood and cortex, it would appear that the surface of the stems was practically smooth. The coaly investment on _Eucalamites_ casts varies considerably in thickness[814]; it is very unsafe to make use of the thickness of this layer as a test of the breadth of the wood in Calamitean stems. The branch-scars as seen in a surface-view of a stem are situated a little above the nodal lines, while depressions on the pith-casts occur in the slight nodal constriction or immediately above it. Small leaf-scars have been described as occurring on the nodes between the branch-scars in specimens showing the surface features[815].

The species long known as _Calamites cruciatus_ Sternb. is usually taken as the type of the sub-genus _Eucalamites_. Weiss[816] has subdivided this species into several ‘forms,’ which he bases on the number of branch-scars on each node and on other characters; a more extended subdivision of _C. cruciatus_ has recently been made by Sterzel[817], who admits the impossibility of separating the specific forms by means of the data at our disposal, but for purposes of geological correlation he prefers to express slight differences by means of definite ‘forms’ or varieties. The more comprehensive use of the specific name _cruciatus_ as adopted by Zeiller in his _Flore de Valenciennes_[818] is, I believe, the better method to adopt. The specimen shown in fig. 102 affords a good example of a typical _Calamites cruciatus_, it was found in the Middle Coal-Measures near Barnsley, Yorkshire.

_Calamites_ (_Eucalamites_) _cruciatus_ (Sternb.). Fig. 102.

1826. _Calamites cruciatus_, Sternberg[819].
1828. _Calamites cruciatus_, Brongniart[820].
1831. _Calamites alternans_, Germar and Kaulfuss[821].
1837. _Calamites approximatus_, Lindley and Hutton[822].
1877. _Calamodendrofloyos cruciatus_, Grand’Eury[823].
1878. _Calamodendron cruciatum_, Zeiller[824].
1884. _Calamites_ (_Eucalamites_) _cruciatus ternarius_, Weiss[825].
1884. „ „ „ _quaternarius_, Weiss[825].
1884. „ „ „ _genarius_, Weiss[825].
1884. „ „ _multiramis_, Weiss[825].
1888. _Calamites_ (_Calamodendron_) _cruciatus_, Zeiller[826].

This species occurs in the Upper, Middle and Lower Coal-Measures[827]. The casts of the _cruciatus_ type have been found associated with wood possessing the structural features of the sub-genus _Calamodendron_[828], but our knowledge of the structure of the stem, and of the fertile branches of _C. cruciatus_ is very imperfect. A restoration of _Calamites_ (_Eucalamites_) _cruciatus_ is given by Stur[829] in his classic work on the Calamites, but he does not make quite clear the supposed connection with the stems and the fertile shoots of the Asterophyllites type[830] which he describes. Another member of the _Eucalamites_ group, which is better known as regards its foliage-shoots, is _Calamites ramosus_, a species first described by Artis[831] in 1825. Stems of this species have been found in connection with the branches and leaves of the _Annularia_[6] type, bearing _Calamostachys_[832] cones. In all probability pith-casts included in the sub-genus _Eucalamites_ belonged to stems with foliage-shoots and probably also with cones of more than one form.

[Sidenote: NOMENCLATURE.]

In the above account of a few common pith-casts it has been pointed out that there is occasionally satisfactory evidence for connecting certain casts with wood of a particular structure, and with sterile and fertile foliage-shoots of a definite type. It is, however, impossible in many cases to recognise with any certainty the leaf-bearing branches and strobili of the different casts of _Calamites_; it is equally impossible to determine what type of pith-cast or what type of foliage-shoots belongs to petrified stem-fragments in which it is possible to investigate the microscopical features. The scattered and piece-meal nature of the material on which our general knowledge of Calamitean plants is based, necessitates a system of nomenclature which is artificial and clumsy; but the apparent absurdity of attaching different names to fragments, which we believe to be portions of the same genus, is of convenience from the point of view of the geologist and the systematist. As our material increases it will be possible to further simplify the nomenclature for Calamarian plants, but it is unwise to allow our desire for a simpler terminology to lead us into proposals which are based rather on suppositions than on established fact. If it were possible to discriminate between pith-casts of stems having the different anatomical characters designated by the three sub-genera, _Arthropitys_, _Arthrodendron_ and _Calamodendron_, the genus _Calamites_ might be used in a much narrower and probably more natural sense than that which we have adopted. The tests made use of by some authors for separating pith-casts of _Calamodendron_ and _Arthropitys_ stems do not appear to be satisfactory; we want some term to apply to all Calamitean casts irrespective of the anatomical features of the stems, or of the precise nature of the foliage-branches. As used in the present chapter, _Calamites_ stands for plants differing in certain features but possessing common structural characters, which must be defined in a broad sense so as to include types which may be worthy of generic rank, but which for convenience sake are included in a comprehensive generic name. The attempts to associate certain forms of foliage with _Arthropitys_ on the one hand and with _Calamodendron_ on the other, cannot be said to be entirely satisfactory; we still lack data for a trustworthy diagnosis of distinct Calamarian genera which shall include external characters as well as histological features. If we restricted the genus _Calamites_ to stems with an _Arthropitys_ structure and an Asterophyllitean foliage, we should be driven into unavoidable error. Within certain limits it is possible to distinguish generically or even specifically between petrified branches, and we already possess material enough for fairly complete diagnoses founded on internal structure; but it is not possible to make a parallel classification for pith-casts and foliage-shoots. For this reason, and especially bearing in mind the importance of naming isolated foliage-shoots and stem-casts for geological purposes, I believe it is better to admit the artificially wide application of the name _Calamites_, and to express more accurate knowledge, where possible, by the addition of a subgeneric term. In dealing with distinctions exhibited by Calamitean stems it may be advisable to make use of specific names, but we must keep before us the probability of the pith-cast and petrified stem-fragment of the same plant receiving different specific names. If the structural type is designated by a special sub-genus, this will tend to minimise the anomaly of using more than one binominal designation for what may be the same individual.

[Sidenote: CALAMITES AND EQUISETUM.]

The following summary may serve to bring together the different generic and subgeneric terms which have been used in the foregoing account of _Calamites_.

CALAMITES.

Subgenera having reference to the method of branching as seen in casts or impressions of the stem-surface or in pith-casts.

_Calamitina_,
_Eucalamites_,
_Stylocalamites_.

Subgenera founded on anatomical characters in stems and branches.

_Arthropitys_,
_Calamodendron_,
_Arthrodendron_ (new sub-genus substituted for _Calamopitys_).

Genus proposed for roots of _Calamites_ before their real nature was recognised. The name refers to anatomical characters.

_Astromyelon._

Genera of which some species, if not all, are the leaf-bearing branches of _Calamites_.

_Calamocladus_ (including _Asterophyllites_),
_Annularia_.

Generic names applied to strobili belonging to _Calamites_.

_Calamostachys_,
_Palaeostachya_,
_Macrostachya_,
_etc._

Genus including impressions of Calamite roots.

_Pinnularia._

IV. Conclusion.

A brief sketch of the main features of _Calamites_ suffices to bring out the many points of agreement between the arborescent Calamite plants and the recent Equisetums. The slight variation in morphological character among the present-day Horse-tails, contrasts with the greater range as regards structural features among the types included in _Calamites_. The Horse-tails probably represent one of several lines of development which tend to converge in the Palaeozoic period; the Calamite itself would appear to mark the culminating point of a certain phylum of which we have one degenerate but closely allied descendant in the genus _Equisetum_. We shall, however, be in a better position to consider the general question of plant-evolution after we have made ourselves familiar with other types of Palaeozoic plants. Grand’Eury’s[833] striking descriptions of forests of Calamites in the Coal-Measures of central France, enable us to form some idea of the habit of growth of these plants with their stout branching rhizomes and erect aerial shoots.

By piecing together the evidence derived from different sources we may form some idea of the appearance of a living Calamite. A stout branching rhizome ascended obliquely or spread horizontally through sand or clay, with numerous whorls or tufts of roots penetrating into swampy soil. From the underground rhizome strong erect branches grew up as columnar stems to a height of fifty feet or more; in the lower and thicker portions the bark was fissured and somewhat rugged, but smoother nearer the summit. Looking up the stem we should see old and partially obliterated scars marking the position of a ring of lateral branches, and at a higher level tiers of branches given off at regular or gradually decreasing intervals, bearing on their upper portions graceful green branchlets with whorls of narrow linear leaves. On the younger parts of the main shoot rings of long and narrow leaves were borne at short intervals, several leaf-circles succeeding one another in the intervals between each radiating series of branches. On some of the leaf-bearing branchlets long and slender cones would be found here and there taking the place of the ordinary leafy twigs. Passing to the apical region of the stem the lateral branches given off at a less and less angle would appear more crowded, and at the actual tips there would be a crowded succession of leaf-segments forming a series of overlapping circles of narrow sheaths with thin slender teeth bending over the apex of the tree.

Thus we may feebly attempt to picture to ourselves one of the many types of Calamite trees in a Palaeozoic forest, growing in a swampy marsh or on gently sloping ground on the shores of an inland sea, into which running water carried its burden of sand and mud, and broken twigs of Calamites and other trees which contributed to the Coal Period sediments. The large proportions of a Calamite tree are strikingly illustrated by some of the broad and long pith-casts occasionally seen in Museums; in the Breslau Collection there is a cast of a stem belonging to the sub-genus _Calamitina_, which measures about 2 m. in length and 23 cm. in breadth, with 36 nodes. In the Natural History Museum, Paris, there is a cast nearly 2 metres long and more than 20 cm. wide, which is referred to the sub-genus _Calamodendron_.

E. ARCHAEOCALAMITES.

In the Upper Devonian and Culm rocks casts of a well-defined Calamitean plant are characteristic fossils; stems, leaf-bearing branches, roots and cones have been described by several authors, and the genus _Archaeocalamites_ has been instituted for their reception. Although this genus agrees in certain respects with _Calamites_, and as recent work has shown this agreement extends to internal structure, it has been the custom to regard the Lower Carboniferous and Devonian plants as genetically distinct. The surface features of the stem-casts, the form of the leaves, and apparently the cones, possess certain distinctive characters which would seem to justify the retention of a separate generic designation.

We may briefly summarise the characteristics of the genus as follows:—

Pith-casts articulated, with very slightly constricted nodes; the internodes traversed by longitudinal ribs slightly elevated or almost flat, separated by shallow grooves. The ribs and grooves are continuous from one internode to another, and do not usually show the characteristic alternation of _Calamites_[834]. Along the nodal line there are occasionally found short longitudinal depressions, probably marking the points of origin of outgoing bundles. Branches were given off from the nodes without any regular order; a pith-cast may have branch-scars on many of the nodes, or there may be no trace of branches on casts consisting of several nodes. The leaves[835] are in whorls; in some cases they occur as free, linear, lanceolate leaves, or on younger branches they are long, filiform and repeatedly forked. The structure of the wood agrees with that of some forms of _Arthropitys_. The strobili consist of an articulated axis bearing whorls of sporangiophores, and each sporangiophore has four sporangia. Our knowledge of the fertile shoots is, however, very imperfect.

Renault[836] has recently described the structure of the wood in some small silicified stems of _Archaeocalamites_ from Autun. A large hollow pith is surrounded by a cylinder of wood consisting of wedge-shaped groups of xylem tracheids associated with secondary medullary rays; at the apex of each primary xylem group there is a carinal canal. The primary medullary rays appear to have been bridged across by bands of xylem at an early stage of secondary thickening, as in the Calamite of fig. 83, _D._

From a specimen in the Woodwardian Museum, Cambridge. From the
Carboniferous limestone of Northumberland. ½ nat. size.]

Our knowledge of the cones of _Archaeocalamites_ is far from satisfactory. Renault[837] has recently described a small fertile branch bearing a succession of verticils of sporangiophores; each sporangiophore stands at right angles to the axis of the cone and bears four sporangia, as in _Calamostachys_. It is not clear how far there is better evidence than that afforded by the association of the specimen with pith-casts of stems, for referring this cone to _Archaeocalamites_, but the association of vegetative and fertile shoots certainly suggests an organic connection. The cone described by the French author agrees with _Equisetum_ in the absence of sterile bracts between the whorls of sporangiophores. It is an interesting fact that such a distinctly Equisetaceous strobilus is known to have existed in Lower Carboniferous rocks.

Stur[838] has also described _Archaeocalamites_ at considerable length; he gives several good figures of stem-casts and foliage-shoots bearing long and often forked narrow leaves. The same writer describes specimens of imperfectly preserved cones in which portions of whorls of forked filiform leaves are given off from the base of the strobilus[839]. Kidston[840] published an important memoir on the cones of _Archaeocalamites_ in 1883, in which he advanced good evidence in support of the view that certain strobili, which were originally described as Monocotyledonous inflorescences, under the generic name _Pothocites_[841], are the fertile shoots of this Calamarian genus. Kidston’s conclusions are based on the occurrence on the _Pothocites_ cones, of leaves like those of _Archaeocalamites_, on the non-alternation of the sporangiophores of successive whorls, and on the close resemblance between his specimens and those described by Stur. Good specimens of the cones, formerly known as _Pothocites_, may be seen in the Botanical Museum in the Royal Gardens, Edinburgh; as they are in the form of casts without internal structure it is difficult to form a clear conception as to their morphological features.

The fossils included under _Archaeocalamites_ have been referred by different authors to various genera, and considerable confusion has arisen in both generic and specific nomenclature. The following synonomy of the best known species, _A. scrobiculatus_ (Schloth.) illustrates the unfortunate use of several terms for the same plant.

_Archaeocalamites scrobiculatus_ (Schloth.). Fig. 103.

1720. _Lithoxylon_, Volkmann[842].
1820. _Calamites scrobiculatus_, Schlotheim[843].
1825. _Bornia scrobiculata_, Sternberg[844].
1828. _Calamites radiatus_, Brongniart[845].
1841. _Pothocites Grantoni_, Paterson[846].
1852. _Calamites transitionis_, Göppert[847].
—— _Stigmatocanna Volkmanniana_, _ibid._
—— _Anarthrocanna tuberculata_, _ibid._
—— _Calamites variolatus_, _ibid._
—— _C. obliquus_, _ibid._
—— _C. tenuissimus_, _ibid._
—— _Asterophyllites elegans_, _ibid._
1866. _Calamites laticulatus_, Ettingshausen[848].
—— _Equisetites Göpperti_, _ibid._
—— _Sphenophyllum furcatum_, _ibid._
1873. _Asterophyllites spaniophyllus_, Feistmantel[849].
1880. _Asterocalamites scrobiculatus_, Zeiller[850].

For other lists of synonyms reference may be made to Binney[851], Stur[852], Kidston[853] and other authors.

Some of the best specimens of this species are to be seen in the Museums of Breslau and Vienna, which contain the original examples described by Göppert and Stur. An examination of the original specimens, figured by Göppert under various names, enables one to refer them with confidence to the single species, _Archaeocalamites scrobiculatus_. The generic name _Archaeocalamites_, which has been employed by some authors, was suggested by Schimper[854] in 1862, as a subgenus of _Calamites_, on account of the occurrence of a deeply divided leaf-sheath, attached to the node of a pith-cast, which seemed to differ from the usual type of Calamitean leaf. The specimens described by Schimper are in the Strassburg Museum; the leaf-sheath which he figures is not very accurately represented.

The example given in fig. 103 shews very clearly the continuous course of the ribs and grooves of the pith-cast. Each rib is traversed by a narrow median groove which would seem to represent the projecting edge of some hard tissue in the middle of each principal medullary ray of the stem. The specimen was found in a Carboniferous limestone quarry, Northumberland; there is a similar cast from the same locality in the Museum of the Geological Survey.

_Affinities of_ Archaeocalamites.

This genus agrees very closely with _Calamites_ both in the anatomical structure of the stem and in the verticillate disposition of the leaves. The strobili appear to be Equisetaceous in character, and there is no satisfactory evidence of the existence of whorls of sterile bracts in the cone, such as occur in _Calamostachys_ and in other Calamitean strobili. The continuous course of the vascular bundles of the stem from one internode to the next is the most striking feature in the ordinary specimens of the genus; but it sometimes happens that the grooves on a pith-cast shew the same alternation at the node as in _Calamites_. This is the case in a specimen in the Göppert collection in the Breslau Museum, and Feistmantel[855] has called attention to such an alternation in specimens from Rothwaltersdorf. In the true _Calamites_, on the other hand, the usual nodal alternation of the vascular strands is by no means a constant character[856]. Stur[857], Rothpletz[858], and other authors have pointed out the resemblance of _Archaeocalamites_ to _Sphenophyllum_. The deeply divided leaves of some Sphenophyllums and those of _Archaeocalamites_ are very similar in form; and the course of the vascular strands in _Sphenophyllum_ may be compared with that in _Archaeocalamites_. But the striking difference in the structure of the stele forms a wide gap between the two genera. We have evidence that the Calamites and Sphenophyllums were probably descended from a common ancestral stock, and it may be that in _Archaeocalamites_, some of the _Sphenophyllum_ characters have been retained; but there is no close affinity between the two plants.

On the whole, considering the age of _Archaeocalamites_ and the few characters with which we are acquainted, it is probable that this genus is very closely related to the typical _Calamites_, and may be regarded as a type which is in the direct line of development of the more modern Calamite and the living _Equisetum_. Weiss[859] includes _Archaeocalamites_ as one of his subgenera with _Calamitina_ and others, and it is quite possible that the genus has not more claim to stand alone than other forms at present included in the comprehensive genus _Calamites_.

The student will find detailed descriptions of this genus in the works which have been referred to in the preceding pages.