The genus Bactryllium is often quoted in text-books as a probable example of a Triassic diatom. It was first described by Heer[268] from the Trias of Switzerland and North Italy, also from the neighbourhood of Heidelberg, and regarded as an extinct member of the Diatomaceae. Heer defined the genus as follows:
“Small bodies, with parallel sides, rounded at either end, the surface traversed by one or two longitudinal grooves.”
(fig. 32, C.) Several species have been figured by Heer from beds of Muschelkalk, Keuper and Rhaetic age. He describes the wall as thick and firm (fig. 32, C. ii.) and probably composed of silica, with a hollow interior. The specimen shown in fig. 32, C. was found in the Rhaetic beds, and named by Heer Bactryllium deplanatum; it has a length of 4·5 mm.; the surface is transversely striated and traversed by a single longitudinal groove. Stefani[269] has given reasons in favour of removing Bactryllium from the plant to the animal kingdom; he points out that the specimens are too large for diatoms, and moreover that they are asymmetrical in form and possessed a calcareous and not a siliceous shell. He would place the fossil among the Pteropods, comparing it with such genera as Cuvierina and Hyalaea. In view of Stefani’s opinion we cannot attach any importance to this supposed diatom, especially as it has generally been regarded as at best but an unsatisfactory genus.
B. CHLOROPHYCEAE (Green Algae).
Thallus unseptate, having the form of a vesicle or a variously branched coenocyte, which may or may not be encrusted with carbonate of lime, or of filaments composed of cells containing a single nucleus, or of cells in which more than one nucleus occurs; in other instances consisting of a plate of cells or a cell-mass. Asexual reproduction by zoospores and other reproductive cells; sexual reproduction by means of the conjugation of similar gametes or by the fertilisation of a typical egg-cell by a motile spermatozoid.
This family of algae is represented at the present day by numerous and widely distributed marine and fresh-water genera, as well as by genera growing in moist air or as endophytes in the tissues of higher plants[270].
Seeing how very few fossil forms have been described which have any claim to inclusion in this subdivision of the Algae, it is unnecessary to enumerate or define the various families of the Chlorophyceae. It is true that many species have been figured as examples of different genera of green algae, but few of these possess any scientific value. There is, however, one division of the Chlorophyceae, the Siphoneae, which must be treated at some length on account of its importance from a palaeobotanical and geological point of view.
a. Siphoneae.
Thallus consisting of simple or branched cells very rarely divided by septa, and containing many nuclei. In certain genera the branches form a pseudoparenchymatous tissue by their repeated branching, and as a result of the intimate felting together of the branched cells. Reproduction is effected either by the conjugation of similar gametes or by the fertilisation of an egg-cell.
Vaucheria and Botrydium are two well-known British genera of this order, but most of the recent representatives live in tropical and subtropical seas. The most striking characteristic feature of this division of the Chlorophyceae is the fact that the thallus of a siphoneous alga consists of an unseptate coenocyte; the plant may be extremely small and simple, or it may reach a length of several inches, but in all cases the body does not consist of more than one cell or coenocyte.
From a palaeontological standpoint the Siphoneae are of exceptional interest. It is impossible to do more than refer to a few of the living and fossil genera. There are numerous fossil representatives already known, and there can be little doubt that further research would be productive of valuable results.
As examples of the order, a few genera may be described belonging to the three families Caulerpaceae, Codiaceae, and Dasycladaceae.
α. Caulerpaceae.
Thallus unseptate, showing an extraordinary variation in the external differentiation of the plant-body. Reproduction is effected by means of detached portions of the parent plant.
The genus Caulerpa, represented by a few species in the Mediterranean and by many tropical forms, has already been alluded to as a striking example of a plant which appears under a great many different forms[271]. As a recent writer has said, “Nature seems to have shown in this genus the utmost possibilities of the siphoneous thallus[272],” Fragments of coniferous twigs, the tracks and burrows of various animals and other objects have been described by several authors as fossil species of Caulerpa. As an illustration of the identification of a very doubtful fossil as a species of Caulerpites, reference may be made to such a form as C. cactoides Göpp.[273] from Silurian and Cambrian rocks. There are several examples of this fossil in the Brussels Museum which probably owe their origin to some burrowing animal, and may be compared with Zeiller’s figures of the tunnels made by the mole-cricket (fig. 30, 4)[274].
Mr Murray, of the British Museum, has recently described what he regards as a trustworthy example of a fossil Caulerpa from the Kimeridge Clay near Weymouth[275]. Specimens of the fossil were first figured in a book on the geology of the Dorset coast as casts of an equisetaceous plant[276].
To this fossil Murray has assigned the name Caulerpa Carruthersi, and given to it a scientific diagnosis. The best specimens have the form of a slender central axis, giving off at fairly regular intervals whorls of short and somewhat clavate branches; they bear a superficial resemblance to such a recent species as Caulerpa cactoides Ag. An examination of several examples of this fossil leads me to express the opinion that there is not sufficient reason for assigning to them the name of a recent genus of algae[277]. To use the generic name of a recent plant without following the common custom of adding on the termination “ites” (i.e. Caulerpites) is as a general rule to be avoided in dealing with fossil forms; and there are, I believe, no satisfactory grounds for referring to these fossils as trustworthy examples of a Mesozoic alga.
In the present case I am disposed to regard the Caulerpa-like casts as of animal rather than plant origin. The clavate branches have the form of very deep moulds in the hard brown rock which have been filled in with blue mud. It is hardly conceivable that the branches of a soft watery plant such as Caulerpa could leave more than a faint impression on an old sea-floor. The specimens occur in different positions in the matrix of the rock and they are not confined to the lines of bedding; in none of the examples is there any trace of carbonaceous matter in association with the deep moulds. On the whole, then, this Kimeridge fossil cannot, I believe, be accepted as an authentic example of a Mesozoic Caulerpa.
It is not improbable that some of the supposed fossil algae may be casts of egg-cases or spawn-clusters of animals. In Ellis’ Natural History of the Corallines[278] there is a drawing representing a number of disc-like ovaries attached to a tough ligament, and referred to the mollusc Buccinum, which bears a certain resemblance to the Weymouth fossil. A similar body is figured by Fuchs[279] in an important memoir on supposed fossil algae.
It is not suggested that the Caulerpa Carruthersi of Murray should be regarded as the cast of some molluscan egg-case attached to a slender axis, but it is important to bear in mind the possibility of matching such extremely doubtful fossils with other organic bodies than the thallus of a Caulerpa. In an example of an egg-case in the Cambridge Zoological Museum, referred to a species of Pyrula, there is a hard, long and slender axis, bearing a series of semicircular chambers divided into radial compartments. The whole is hard and horny and might well be preserved as a fossil.
β. Codiaceae.
The members of this Order present a considerable diversity of form as regards the shape of the plant-body; the thallus of some species is encrusted with carbonate of lime. The order is widely distributed in tropical and temperate seas.
Among the recent genera Penicillus and Codium may be chosen as important types from the point of view of fossil representatives.
The thallus of Codium consists of a spongy mass of tubular cell-branches which are differentiated into two fairly distinct regions, an outer peripheral layer in which the branches have long club-shaped terminations, and an inner region consisting of loosely interwoven filaments.
Codium Bursa L. and C. tomentosum Huds. are two well-known British species, the former presents the appearance of a spongy ball of cells, and in the latter the thallus is divided up into dichotomously forked branches[280]. In this genus the thallus is not encrusted with carbonate of lime, at least in recent species.
Rothpletz[281] instituted this genus for certain small spherical or oval bodies varying from 1 mm. to 2 cm. in diameter, which have been found on crinoid stems or shell fragments of Triassic age. Each spherical body consists of dichotomously branched single-celled filaments, between 50 and 100µ in breadth, and from 300–500µ in height. The tubular cavities occasionally swell out into spherical spaces which are regarded by Rothpletz as sporangia.
There is not sufficient evidence that Sphaerocodium Bornemanni Roth. has been correctly referred to the Codiaceae. The sporangia-like swellings described by the author of the species are not by any means conclusive as characters of important taxonomic value. Figure 37, D, illustrates the general structure of the fossil as seen in a transverse section of one of the calcareous grains.
Like Girvanella, which has been referred by some writers to the Siphoneae, Sphaerocodium occurs in the form of oolitic grains. In the Triassic Raibler and St Cassian beds of the Tyrol, as well as in rocks of Rhaetic age in the Eastern Alps, it makes up large masses of limestone. Rothpletz compares the structure of this genus with that of the recent alga Codium adhaerens Ag., but it is wiser to regard such tubular structures as Girvanella, Siphonema[282] and Sphaerocodium as closely allied organisms, which are probably algae, but too imperfectly known to be referred to any particular family.
The recent genus Penicillus is one of those algae formerly included among animals. Fig. 33, O, has been copied from a drawing of a species of Penicillus given by Lamouroux[283] under the generic name of Nesea in his treatise on the genera of Polyps published in 1821. He describes the genus as a brush-like Polyp with a simple stem.
The thallus consists of a stout stem terminating in a brush-like tuft of fine dichotomously-branched filaments. The apical branches are divided by regular constrictions into short oval or rod-like segments which may be encrusted with carbonate of lime. A few of the segments from the terminal tuft of a recent Penicillus are shown in fig. 35, E. Each of these calcareous segments has the form of an oval shell perforated at each end, and the wall is pierced by numerous fine canals. Penicillus is represented by about 10 recent species, which with one exception live in tropical seas.
The recognition of Penicillus, or a very similar type, in a fossil condition is due to Munier-Chalmas[284]. This keen observer has rendered great service to palaeobotany by directing attention to the calcareous algae in the Paris basin beds, and by proving that many of the fossils from these Tertiary deposits have been erroneously included by previous writers among the Foraminifera[285]. It is greatly to be desired that Prof. Munier-Chalmas may soon publish a monograph on the fossil Siphoneous forms of which he possesses a unique collection.
In his Natural History of Invertebrate Animals, Lamarck described some small oval bodies from the Calcaire Grossier (Eocene) of the Paris basin under the name of Ovulites. He defined them as follows:—“Polypier pierreux, libre, ovuliforme ou cylindracé, creux intérieurement, souvent percé aux deux bouts. Pores très petits, régulièrement disposés à la surface[286].”
The specimens are referred to two species, Ovulites margaritula and O. elongata.
By some subsequent writers[287] these calcareous fossils, like miniature birds’ eggs with a hole at either end, were included among the Zoophytes. Carpenter and others afterwards referred Ovulites to the Foraminifera, and compared the genus with Lagena[288]. The single specimens of Ovulites have a length of 2–6 mm. At each end there is usually a fairly large and somewhat irregular hole (fig. 35, F), and in some rarer cases there may be two apertures at the broader end of an Ovulite. A good example of Ovulites margaritula with two pores at the broader end is figured by Michelin[289]. The surface of the shell when seen under a low magnifying power appears to be covered over with regularly arranged circular pores, which are the external openings of fine canals (fig. 33, L).
In 1878 Munier-Chalmas expressed the opinion, which was supported by strong evidence, that Ovulites should be referred to the siphoneous algae[290]. He regarded it as generically identical with Penicillus (Coralliodendron, Kützing). It has already been pointed out that in Penicillus the apical tuft of filaments is partially calcareous (fig. 33, O)[291]. The individual calcareous segments agree almost exactly with the fossil Ovulites. As a rule the Ovulites occur as separate egg- or rod-like bodies, but Munier-Chalmas informs me that occasionally two or three have been found joined end to end in their natural position. The terminal holes in the fossil specimens represent the apertures left after the detachment of the calcareous segments from the uncalcified filaments of the alga. The segments with two holes at the broader end were no doubt situated at the base of dichotomising branches as shown in fig. 33, K. The restoration of Ovulites, shown in fig. 33, K, bears a striking resemblance to the figure of an Australian Penicillus given by Harvey in his Phycologia Australica[292].
It is probable that these Eocene forms agreed closely in habit with the recent species of Penicillus. The portions preserved as fossils are segments of the filaments which probably formed a terminal brush of fine branches supported on a stem. The retention of the original generic name Ovulites is on the whole better than the inclusion of the fossil species in the recent genus. The Tertiary species lived in warm seas of the Lower and Middle Eocene of England, Belgium, France and Italy.
An example of an Eocene species of Halimeda has been recorded by Fuchs from Greifenstein under the name of Halimeda Saportae[293]. The impression has the form of a branched plant consisting of wedge-shaped or oval segments, and there is a close resemblance to the thallus of a recent Halimeda, e.g. H. gracilis Harv. It is not improbable that Fuchs’ determination is correct, but without more definite evidence than is afforded by a mere impression it is a little rash to make use of the recent generic name.
γ. Dasycladaceae.
In this family of Siphoneae are included a number of genera represented by species living in tropical and subtropical seas.
The thallus consists of an elongated axial cell fixed to the substratum by basal rhizoids, and bearing whorls of lateral appendages of limited growth which may be either simple or branched. Many of the lateral branches bear sporangia or spores. The thallus is in many species encrusted with carbonate of lime.
The two genera Acetabularia and Cymopolia may be briefly described as recent types which are represented by trustworthy fossil forms.
With the exception of A. mediterranea Lamx. (fig. 34) the few living species of this genus are confined to tropical seas.
The habit of Acetabularia is well illustrated by the photograph of a cluster of plants of A. mediterranea Lamx.[294] reproduced in fig. 34. The thallus consists of a delicate stalk attached to the substratum by a tuft of basal holdfasts, and expanded distally into a small circular disc 10–12 mm. in diameter and more or less concave above. This terminal cap is made up of a number of laterally fused appendages given off from the upper part of the stalk in the form of a crowded whorl. The whole thallus resembles a small and long-stalked calcareous fungus. In each radially elongated compartment of the fertile cap (fig. 33, I) there are several sporangia (gametangia) developed; these eventually open and produce numerous ciliated gametes which give rise to zygospores by conjugation. Fig. 33, I, represents the cap of an Acetabularia in radial section and surface-view; the two radial compartments seen in section contain the elliptical gametangia; the circular markings at the base of the figure are scars of sterile deciduous branches.
The whole plant is unicellular, each chamber in the disc being in open communication with the stem of the plant.
In a recent monograph on the Acetabularieae, Solms-Laubach[295] has described a new type of these algae which is of special importance from the point of view of the past history of the family. Möbius described an example of Acetabularia in 1889 under the name A. Schencki; this species has since been placed in D’Archiac’s genus Acicularia[296]. Acicularia Schencki[297] bears a close resemblance as regards external form to Acetabularia mediterranea. In the latter species the walls of the terminal disc compartments are calcified, and the cavity of each of the laterally fused members contains numerous free spores; in Acicularia, the cavity of each disc-ray is occupied by a calcareous substance in the form of a spicule containing numerous cavities in each of which is a single sporangium. A single spicule is seen in fig. 33, H, showing the spherical pockets in which the sporangia were originally situated. This species, Acicularia Schencki, has been recorded from Martinique, Guadeloupe, Brazil, and a few other places.
The genus Acicularia was founded by D’Archiac for certain minute calcareous spicules found in the Eocene sands (Calcaire Grossier) of the Paris basin. D’Archiac describes one species, Acicularia pavantina, which he defines as follows:—“Polypier aciculaire, élargi, et légèrement comprimé à sa partie supérieure, qui est échancrée au milieu. Surface couverte de petits pores simples, nombreux, disposés irrégulièrement[298].” The same species is figured also in Michelin’s Iconographie Zoophytologique, and described as an organism of which the exact zoological position is uncertain[299]. After these fossils had been placed in various divisions of the animal kingdom, Carpenter[300] described several specimens as portions of foraminifera. Finally, Munier-Chalmas removed Acicularia to the plant kingdom, and “with rare divination” placed the genus among the Acetabularieae. The history of our knowledge of the true nature of Acicularia is of unusual interest. Some of the specimens of this genus figured in Carpenter’s monograph have the form of imperfect long and narrow bodies tapering to a point at one end and broad at the other (fig. 33, F and G); they are joined together laterally and pitted with numerous small cavities. From the resemblance of such specimens to a fragment of the terminal fertile disc of the recent Acetabularias, Munier-Chalmas referred the fossils to this type of algae. In the living species which were then known the radiating chambers of the disc contained loose sporangia, without any calcareous matrix filling the cavity of the chambers. In the fossil Acicularias, on the other hand, the manner of preservation of the pitted calcareous spicules pointed to the occurrence of sporangia embedded in cavities in a calcareous matrix. Subsequent to Munier-Chalmas’ somewhat daring conclusions as to the relation of Acicularia to Acetabularia, Solms-Laubach found that the species originally described by Möbius as Acetabularia Schencki from Guadeloupe presented exactly those characters in which the fossil specimens differ from Acetabularia. The genus Acicularia formerly restricted to fossil species is now applied also to this single living species Acicularia Schencki.
The genus is thus defined by Solms-Laubach:—
“Discus fertilis terminalis e radiis inter se conjunctis formatus, coronis et inferiore et superiore praeditis, sporae massa mucosa calce incrustata coalitae, pro radio spiculam solidam cuneatam formantes[301].”
As Solms-Laubach points out in his recent monograph, Munier-Chalmas’ conjecture, “which had little to support it in the fossil material, has been more recently proved true in the most brilliant fashion by the discovery of a living species of this genus.”
1. Acicularia Andrussowi Solms[302]. Fig. 33, C and D. This species was first described by Andrussow[303] as Acetabularia miocenica from the Crimea. It occurs in Miocene rocks south of Sevastopol, and, with Ostrea and Pecten, forms masses of white limestone.
In each sporangial ray of the disc the cavity contains a calcareous spicula bearing spore cavities in four rows. “Round each spore-cavity there is a circular zone which stands out, when viewed in reflected light, through its white colour against the central mass of the spicule, though a sharp contour is not visible[304].” Fig. 33, C, is taken from a somewhat diagrammatic sketch by Andrussow; it shows ten of the fertile rays of the disc. The thick walls of the chambers are seen in the two lowest rays, and in the next two rays the spore-cavities are represented. A more accurate drawing, from Solms-Laubach’s memoir, is reproduced in fig. 33, D. The calcareous spicule with numerous spore-cavities shown in fig. 33, H, is from a fertile ray of the recent species Acicularia Schencki. This corresponds to the spore-containing calcareous matrix in each ray of the disc of Acicularia Andrussowi Solms. The spicule copied in fig. 33, F from one of Carpenter’s drawings[305] of an Eocene specimen bears the closest resemblance to the recent spicule of fig. 33, H, and emphasizes the very close relationship between the fossil forms and the single rare tropical species.
2. Acicularia miocenica Reuss. Another Tertiary species has been described under this name by Reuss[306] from the Miocene of the Vienna district, from the Leithakalk of Moravia and elsewhere. It agrees very closely with the recent species A. Schencki. A section of one of the spicules of this species is shown in fig. 33, E; the dark patches represent the pockets in the calcareous spicule which were originally occupied by sporangia and spores.
The genus Cymopolia is at present represented by two species, C. barbata (L.) and C. mexicana, Ag., living in the Gulf of Mexico and off the Canary Islands.
Cymopolia and Acetabularia, with several other calcareous algae, are figured by Ellis and other writers as members of the animal kingdom. Ellis speaks of the species of Cymopolia which he figures as the Rosary Bead-Coralline of Jamaica.
Fig. 33, M, has been drawn from a figure published by Ellis in his Natural History of the Corallines published in 1755[307]. The thallus has the form of a repeatedly forked body, of which the branches are divided into cylindrical joints thickly encrusted with carbonate of lime, but constricted and uncalcified at the limits of each segment. A tuft of hairs is given off from the terminal segment of each branch. The axis of each branch of the thallus is occupied by a cylindrical and unseptate cell which gives off crowded whorls of lateral branches. In the lower part of fig. 33, M, the calcareous investment has been removed, and the branches are seen as fine hair-like appendages of the central cell. The branches given off from the constricted portions of the axis are unbranched simple appendages, but the others terminate in bladder-like swellings, each of which bears an apical sporangium. The sporangia are surrounded and enclosed by the swollen tips of four to six branches which spring from the summit of the sporangial branch. Fig. 33, A, represents part of a transverse section through the calcareous outer portion of a branch of Cymopolia; the darker portions or cavities in the calcareous matrix were originally occupied by the lateral branches and sporangia[308].
In Fig. 33, B, the sporangial branch with the terminal sporangium and three of the investing branches are more clearly shown, the surrounding calcareous investment and the thallus having been removed by the action of an acid.
In a transverse section of a branch from which the organic matter had been removed, and only the calcareous matrix left, one would see a central circular cavity surrounded by a thick calcareous wall perforated by radially disposed canals and containing globular cavities; the canals and cavities being occupied in the living plant by branches and sporangia respectively.
The two circular cavities shown in the figure mark the position of the sporangia which are borne on branches with somewhat swollen tips. From the summit the left-hand sporangial branch shown in fig. 33, A, three of the secondary branches are represented by channels in the calcareous matrix; the two black dots on the face of the sporangiophore being the scars of the remaining two secondary branches.
By the lateral contact of the swollen ends of the ultimate branches enclosing the sporangia the whole surface of the thallus, when examined with a lens, presents a pitted appearance. Each pit or circular depression (fig. 33, N) marks the position of the swollen tip of a branch.
This form of thallus represents a type which is met with in several members of the Dasycladaceae. It would carry us beyond the limits of a short account to describe additional recent genera which throw light on the numerous fossil species. For further information as to the recent members of the family, the student should refer to Murray’s Seaweeds[309], and for a more detailed memoir on the group to Wille’s recent contribution to the Pflanzenfamilien[310] of Engler and Prantl. Among the various special contributions to our knowledge of the Dasycladaceae, those by Munier-Chalmas[311], Cramer[312], Solms-Laubach[313], and Church[314], may be mentioned.
The publication of a short preliminary note by Prof. Munier-Chalmas in the Comptes Rendus for 1877 was the means of calling attention to the exceptional importance of the calcareous Siphoneae as algae possessing an interesting past history, of which satisfactory records had been preserved in rocks of various ages. Decaisne had pointed out in 1842 that certain marine organisms previously regarded as animals should be transferred to the plant kingdom. Such seaweeds as Halimeda, Udotea, Penicillus and others were thus assigned to their correct position. Many fossil algae belonging to this group continued to be dealt with as Foraminifera until Munier-Chalmas demonstrated their true affinities. In Gümbel’s monograph on the so-called Nullipores found in limestone rocks, published in 1871[315], several examples of siphoneous algae are included among the fossil Protozoa.
In recent years there have been several additions to an already long list of fossil Siphoneae. In addition to the numerous and well-preserved specimens, representing a large number of generic and specific forms, which have been collected from the Eocene of the Paris basin, there is plenty of evidence of the abundance of the members of the Dasycladaceae in the Triassic seas. In the Triassic limestones of the Tyrol, as well as in other regions, the calcareous bodies of siphoneous algae have played no inconsiderable part as agents of rock-building[316]. Genera have been recorded from Silurian and other Palaeozoic horizons, and there is no doubt that the Verticillate Siphoneae of to-day are the remnants of an extremely ancient family, which in former periods was represented by a much more widely distributed and more varied assemblage of species. There is probably no more promising field of work in the domain of fossil algae than the further investigation of the numerous forms included in Munier-Chalmas’ class of Siphoneae Verticillatae. A brief description of a few genera from different geological horizons must suffice to draw attention to the character of the data for a phylogenetic history of this group.
The fossil examples of the genus Cymopolia (Polytrypa) were originally described by Defrance[317] in the Dictionnaire des Sciences Naturelles as small polyps under the generic name Polytrypa.
In the Eocene sands of the Paris basin there have been found numerous specimens of short, calcareous tubes which Munier-Chalmas has shewn are no doubt the isolated segments of an alga practically identical with the recent Cymopolia. A section[318] through one of the fossil segments presents precisely the same features as those which are represented in fig. 33, A. The habit of the Eocene alga and its minute structure were apparently almost identical with those of the recent species, Cymopolia barbata. The two drawings of Cymopolia reproduced in fig. 33, A and B, have been copied from Munier-Chalmas’ note in the Comptes Rendus[319]; the corresponding figures given by this author of the Eocene species (Cymopolia elongata Deb.) are practically identical with figs. A and B, and show no points of real difference. The segments of the thallus of the fossil species, as figured by Defrance[320], appear to be rather longer than those of the recent species. The calcareous investment of the axial cell of the thallus was traversed by regular verticils of branches or ‘leaves’; the central branch of each whorl terminates in an oval sporangial cavity, exactly as in fig. 33, A and B; and from the top of this branch there is given off a ring of slender prolongations which terminate on the surface of the calcareous tube as regularly disposed depressions, which were no doubt originally occupied by their swollen distal ends as in the recent species.
This generic name was proposed by Stolley for certain branched and curved tubes found in Silurian boulders from the North German drift[321]. The tubes have a diameter of ·5–1 mm., and are perforated by radial canals which probably mark the position of verticils of branches given off at right angles to the central axis. The surface of the tubes is divided into regular hexagonal areas.
The resemblance of these Silurian fossils to Diplopora and other genera favours their inclusion in the Verticillate Siphoneae.
The fossils included in this genus were first described by Sandberger from the middle Devonian rocks of the Eifel, and referred by him to the animal kingdom. More recently Deecke has suggested the removal of the genus to the calcareous Siphoneae, and such a view appears perfectly reasonable, although without more data it is not possible to speak with absolute certainty.
Sycidium melo. (Sandb.) Fig. 32, B. The specimen represented in fig. 32, B (i), (ii), drawn from Deecke’s figures[322], has the form of a small oval calcareous body, 1 mm. in transverse diameter and 1–1·3 mm. in longitudinal diameter. It is pointed at one end and flattened at the other. At the flatter end there is a circular depression, continued into a funnel-shaped cavity, and on the walls of this cavity there are 18–20 radially disposed ribs, which extend over the surface of the whole body. A series of transverse ribs intersects the vertical ribs at right angles. The calcareous wall is perforated by numerous whorls of circular pores, and the internal cavity is a simple undivided space. Each of these oval bodies (fig. 33, B) is probably the segment of a thallus, and the perforations in the wall may have been originally occupied by lateral prolongations from the unseptate axial cell of the thallus. Sycidium bears a fairly close resemblance to the Tertiary Ovulites.
This genus of algae is characteristic of Triassic rocks, and is especially abundant in Muschelkalk and Lower Keuper limestones of the Alps, Silesia, and elsewhere. The thallus, or rather the calcareous portion of the thallus, has the form of a thick-walled tube, with a diameter of about 4 mm., and occasionally reaching a length of 50 mm. At one end the tube has a rounded and closed termination, and the wall is pierced throughout its whole length by regular whorls of fine canals. Diplopora agrees with Cymopolia in its main features.