Chapter VII: Thallophyta (2)

How then are we to recognise the traces of ancient algae? There is no golden rule, and we must admit the difficulty of separating real fossil algae from markings made by animal or mechanical agency. The presence of a carbonaceous film is occasionally a help, but its occurrence is no sure test of plant origin, nor is its absence a fatal objection to an organic origin. While being fully alive to the small value of external resemblance, and to the numerous agents which have been shown to be capable of producing appearances indistinguishable from plant impressions, we must not go too far in a purely negative direction.

An important contribution to the subject of fossil algae has lately appeared by Prof. Rothpletz[241]. He deals more particularly with the much discussed Flysch[242] Fucoids of Tertiary age, and while refusing to accept certain examples as fossil algae, he brings forward weighty arguments in favour of including several other forms among the algae. He is of opinion that most of the main divisions of the algae are represented among the Flysch Fucoids, but considers that the Phaeophyceae are the most numerous.

Rothpletz’s work is chiefly interesting as illustrating the application of microscopic examination and chemical analysis to the determination of fossil algae. Although he makes out a good case in favour of restoring many of the Tertiary fossils to the plant kingdom, the material at his disposal does not admit of satisfactory botanical diagnosis.

No doubt some of the fossils from the Silurian and Cambrian rocks are true algae, and Nathorst has pointed out that such a species as Hall’s _Sphenothallus angustifolius_[243] may well be an alga. Additional examples might be quoted from Bornemann and other writers, but in view of the attempts which are sometimes made to trace the development of more recent plants to more than doubtful Lower Palaeozoic Algae, one must agree with Nathorst’s opinion,—“Je crois que l’on rend un bien mauvais service à la théorie de l’évolution, en essayant de baser l’arbre généalogique des algues fossiles sur des corps aussi douteux que les Bilobites, Crossochorda, Eophyton, etc.[244]”

There are many carbonaceous impressions on rocks of different ages which it is reasonable to refer to algal origin, and although such are of little or no botanical value, it may be a convenience to refer to them under a definite term. The comprehensive generic name _Algites_[245] has been suggested as a convenient designation for impressions or casts which are probably those of algae.

[Sidenote: SUPPOSED FOSSIL ALGAE.]

Some of the fossils described by Mr Kidston from British Carboniferous rocks as probably algae present an undoubted algal appearance, and might be placed in the genus _Algites_; but in some cases—e.g. _Chondrites plumosa_[246] Kidst. from the Calciferous Sandstone of Eskdale, one feels much more doubtful; in this particular instance the impressions suggest the fine roots of a water-plant.

The statement is occasionally made that the numerous fossil algae and the absence of higher plants in the older strata justify the description of the oldest rocks as belonging to the ‘age of algae.’ Such an assertion rests on an unsound basis, and is rather the expression of what might be expected than what has been proved to be the case. The oldest plants with which we are at all closely acquainted are of such a type as to forcibly suggest that in the lowest fossiliferous rocks we are still very far from the sediments of that age which witnessed the dawn of plant life.

Many of the obscure markings on rock surfaces which have been referred to existing genera of algae or described as new genera, are much too doubtful to be included even under such a comprehensive name as _Algites_. Space does not admit of further reference to determinations of this type which abound in palaeontological literature.

It would be very difficult to produce satisfactory evidence for the algal nature of many of the supposed fossil algae from Cambrian rocks[247]; there has been a special tendency to recognise algal remains in the oldest fossiliferous strata, due in part no doubt to the fallacy that in that period nothing higher than Thallophytes is likely to have existed. The so-called _Phycodes_ referred to by Credner[248] as characteristic of the Cambrian rocks of the Fichtelgebirge (“Phycoden-Schiefer”) is probably of inorganic origin, and comparable to the genus _Vexillum_ of Saporta[249] and other writers, which Solms-Laubach has described as being formed every day in the soft mud of our ponds where local currents are checked by branches and other obstacles[250]. There are several good specimens of _Phycodes_ in the Bergakademie of Berlin and in the Leipzig Museum which, I believe, clearly demonstrate the absence of all satisfactory evidence of an algal origin.

We may next pass to a short description of a few representative types of algae, which may reasonably be classed under definite families, and accepted as evidence possessing some botanical value.

A. DIATOMACEAE (BACILLARIACEAE).

This family occupies a somewhat isolated position among the algae, and is best considered as a distinct subdivision rather than as a family of the Phaeophyceae or Brown algae, with which it possesses as a common characteristic a brown-colouring matter.

Single-celled plants consisting of a simple protoplasmic body containing a nucleus and brown colouring matter (diatomin) associated with the chlorophyll. The cell-wall is in the form of two halves, known as _valves_, which fit into one another like the two portions of a pill-box. The cell-wall contains a large amount of silica, and the siliceous cases of the diatoms are commonly spoken of as the valves of the individual, or the _frustules_. Diatoms exhibit a characteristic creeping movement, and are reproduced by division, also by the development of spores in various forms[251].

The recent members of the family have an exceedingly wide distribution, occurring both in freshwater and in the sea. Owing to the lightness of the frustules, they are frequently carried along in the air, and atmospheric dust falling on ships at sea has been found to contain large numbers of diatoms[252]. The siliceous valves are abundant in guano deposits, and they have been found also in association with volcanic material. Diatomaceous deposits are now being formed in the Yellowstone Park district; “they cover many square miles in the vicinity of active or extinct hot spring vents of the park, and are often three feet, four feet, and sometimes five to six feet thick[253].” The gradual accumulation of the siliceous tests on the floor of a fresh-water lake results in the formation of a sediment consisting in part of pure silica. Such deposits, often spoken of as _kieselguhr_ or _diatomite_, and used as a polishing material, occur in many parts of Britain, marking the sites of dried-up pools or lakes. At the northern end of the island of Skye there occurs an unusually pure deposit of diatomite overlain by peat and turf, and extending over an area of fifty-eight square miles. Many of the individuals in this deposit were in all probability carried into the lake by running water, while others lived in the lake and after death their tests contributed to the siliceous deposit[254]. The late Dr Ehrenberg published numerous papers on diatomaceous deposits in different parts of the world, and in his great work, _Zur Mikrogeologie_[255], he gave numerous and beautifully executed illustrations of such siliceous accumulations. In many of the samples he figures one sees fragments of plant tissues, spores of conifers and ferns, associated with the diatom tests. The occurrence of the pollen grains of coniferous trees in lacustrine and marine deposits is not surprising in view of their abundance in Lake Constance and other lakes. It is stated that the pollen of conifers in the Norwegian fiords plays an important part in the nourishment of the Rhizopod _Saccamina_[256].

[Sidenote: DIATOMACEOUS OOZE.]

In the waters of the ocean diatoms are of frequent occurrence, and very widely distributed. Sir Joseph Hooker records the existence of masses of diatomaceous ooze over a wide area in Antarctic regions[257]. Along the shores of the Victoria Barrier, a perpendicular wall of ice, between one and two hundred feet above sea-level, the soundings were found to be invariably charged with diatom remains, and from the base of the ice-wall there appeared to be in process of formation a bank of these tests stretching north for a distance of 200 miles. The more extended researches conducted during the cruise of the Challenger have clearly proved the enormous accumulations of diatoms now being formed on the ocean-bed[258]. South of latitude 45° S. there is now being built up a vast deposit which may be eventually upraised as a fairly pure siliceous rock. From extreme northern latitudes Nansen has recently recorded the occurrence of these lowly organised plants. He writes,—“I found a whole world of diatoms and other microscopical organisms, both vegetable and animal, living in the fresh-water pools on the Polar drift-ice, and constantly travelling from Siberia to the east coast of Greenland[259].” In warmer latitudes diatoms abound in the surface waters, but there they are associated with numerous other forms of the Plankton vegetation. The waters of the Amazon carry with them into the sea large numbers of fresh-water forms, which are floated out to sea and finally added to the rock-building material which is constantly accumulating on the ocean floor[260]. No definite results have so far been obtained as to the geographical and bathymetrical distribution of marine diatoms.

The enormous number of recent species precludes any attempt to give a description of the better-known forms. It is more important for us to realize how common and widely distributed are the living genera. The hard and almost indestructible valves have been frequently found in a fossil condition, often forming thick and extensive masses of siliceous rock. From diatom-beds now forming in lakes and on the ocean-bed we pass to deposits such as those in Skye and elsewhere, which mark the site of recently dried-up sheets of water, and so to older rocks of Tertiary age formed under similar conditions. Among the many examples of diatomaceous deposits of Tertiary and Cretaceous age mention should be made of those of Berlin, Königsberg, Bilin in Bohemia, and Richmond in Virginia. The diatoms in the beds of Berlin are regarded as fresh-water, and those of Richmond as marine. It has been pointed out by Pfitzer that it is a comparatively easy matter to distinguish between fresh-water and marine forms of diatoms. The diatomaceous rocks of Bilin are known as polishing slates; they attain a thickness of 50 feet. In these, as in many other cases, the deposit has become cemented together as a hard flinty or glassy rock, in which the cementing material was formed by the solution of some of the diatom tests[261]. In many cases in which calcareous and siliceous rocks reveal no direct evidence of organic origin it is probable that they were originally formed by the accumulation of plants of which the structure has been completely obliterated by secondary causes. The genus _Gallionella_ plays an important part in the composition of the Bilin beds. Occasionally impressions of leaves and other organic remains are found associated with the diatoms in the siliceous rocks. In the British Museum (Botanical department) a large block of white powdery rock is exhibited as an example of a diatomaceous deposit of Tertiary age from Australia. It is described as being largely made up of the tests of fresh-water diatoms, such as _Navicula_, _Gomphonema_, _Cymbella_, _Synedra_, and others.

[Sidenote: FOSSIL DIATOMS.]

The abundance of Diatoms in Cretaceous rocks of the Paris basin has recently been recorded by Cayeux[262]; it would seem that these algae had already assumed an important rôle as rock-builders in pre-Tertiary times. Cayeux points out that the silica of these Cretaceous diatomaceous frustules has often been replaced by carbonate of calcium.

In addition to the occurrence of Diatoms in the various diatomaceous deposits, their siliceous tests may occasionally be recognised in argillaceous or other sediments. Shrubsole and Kitton[263] have described several species of Diatoms from the London Clay of Lower Eocene age. In many localities in the London basin the clay obtained from well-sinkings presented the appearance of being dusted with sulphur-like particles of a dark bronze or golden colour which glistened in the sunlight. These yellow bodies have been found to be diatomaceous frustules in which the silica has been replaced by iron pyrites. The genus _Coscinodiscus_ is one of the commonest forms recorded from the London Clay[264].

Without further considering individual examples of diatomaceous rocks we may briefly notice the general facts of the geological history of the family. As Ehrenberg pointed out several years ago, the Tertiary and Cretaceous species of diatoms show a very marked resemblance to living forms. In many cases the species are identical, and the fossil deposits as a whole seem to differ in no special respect from those now being built up.

With the exception of two species of Liassic Diatoms, no trustworthy examples of the Diatomaceae have been found below the Cretaceous series. The oldest known Diatoms were discovered by Rothpletz[265] among the fibres of an Upper Lias sponge from Boll in Württemberg. They occur as small thimble-shaped siliceous tests with coccoliths and foraminifera in the horny skeleton of _Phymatoderma_, a genus formerly regarded as an alga. Rothpletz describes two species which he includes in the genus _Pyxidicula_, _P. bollensis_ and _P. liasica_. This generic name of Ehrenberg is used by Schütt[266] as a subgenus of _Stephanopyxis_.

Seeing how great a resemblance there is between the recent and Cretaceous species, and how many examples there are of Tertiary diatom deposits, it is not a little surprising that the past history of these plants has not been traced to earlier periods. In 1876 Castracane[267], an Italian diatomist, gave an account of certain species of diatoms said to have been found in a block of coal from Liverpool obtained from the English Coal-Measures. The species were found to be identical with recent forms. It is generally agreed that these specimens cannot have been from the coal itself, but that they must have been living forms which had come to be associated with the coal. The late Prof. Williamson spent many years examining thin sections and other preparations of coal from various parts of the world, but he never found a trace of any fossil diatom. There is no apparent reason why diatoms should not be found in Pre-Cretaceous rocks, and the microscopic investigation of old sediments may well lead to their discovery. Prof. Bertrand of Lille, who has devoted himself for some time past to a detailed microscopical examination of coal, informs me that he has so far failed to discover any trace of Palaeozoic diatomaceous tests.

[Sidenote: BACTRYLLIUM.]

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_, _Sycidium melo_ Sandb. (i) Surface view, (ii)
transverse section (after Deecke). _C_, _Bactryllium deplanatum_
Heer. (i) Surface view, (ii) section, showing the thick wall and
hollow interior (after Heer). _D_, Calcareous pebble from a lake in
Michigan. Rather less than nat. size (after Murray).]

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.

_Codium._

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.

_Sphaerocodium._ Fig. 37, D.

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.

_Penicillus._

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.

_Ovulites._ Figs. 33, K, L, and 35, F.

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.

_Halimeda._

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.

_Acetabularia._ Figs. 33, I, and 34.

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.

_Acicularia._ Fig. 33, C–H.

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.

_Cymopolia_. Fig. 33, A, B, M and N.

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.

[Sidenote: PALAEOZOIC SIPHONEAE.]

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.

_Vermiporella._

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.

_Sycidium._ Fig. 32, B.

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_.

_Diplopora._ Fig. 35, A and B.

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.

Fig. 35, A, affords a diagrammatic view of a _Diplopora_ tube, and shews the arrangement of the numerous whorls of canals. In fig. 35, B, a piece of limestone is represented containing several Diploporas cut across transversely and more or less obliquely. In an obliquely transverse section of a tube perforated by horizontal canals the cavities of the canals necessarily appear as holes or discontinuous canals in the substance of the calcareous wall. The manner of occurrence of the specimens points to the abundance of this genus in the Triassic seas, and suggests that the calcareous tubes of _Diplopora_ may have been important factors in the building up of limestone sediments[323]. In many instances no doubt the carbonate of lime of the thallus has been dissolved and recrystallised, and the original form completely obliterated. As in the rocks built up largely of calcareous Florideae (p. 185) which have lost their structure, it is a legitimate inference that some of the limestone rocks which shew no trace of organic structure may have been in part derived from the calcareous incrustation of various algal genera.

_Gyroporella._ Fig. 35, C and D.

In this genus from the Alpine Trias the structure of the calcareous tube is very similar to that in _Diplopora_, but in _Gyroporella_ the canals form less distinct whorls and are closed externally by a small plate, as seen in figs. 35, C and D.

As Solms-Laubach has pointed out, the branch-systems of _Diplopora_, _Gyroporella_ and other older genera are much simpler than in the Tertiary genera _Dactylopora_ and others[324].

A species of _Gyroporella_, _G. bellerophontis_, has recently been described by Rothpletz[325] from Permian rocks in the Southern Tyrol. The thallus is tubular in form and has a diameter of ·5–1 mm.

_Dactylopora._

The genus _Dactylopora_ was founded by Lamarck[326] on some fossil specimens from the Calcaire Grossier and included among the Zoophytes. D’Orbigny afterwards included it among the Foraminifera, and the structure of the calcareous body has been described by Carpenter[327] and other writers on the Foraminifera. In a specimen of _Dactylopora cylindracea_ Lam. from the Paris basin, for which I am indebted to Munier-Chalmas, the tubular thallus measures 4 mm. in diameter; at the complete end it is closed and bluntly rounded. The wall of the tube is perforated by numerous canals, and contains oval cavities which were no doubt originally occupied by sporangia. The shape of the specimens is similar to that of _Diplopora_, but the canals and cavities present a characteristic and more complex appearance, when seen in a transverse section of the wall, than in the older genus _Diplopora_. Gümbel has given a detailed account of this Tertiary genus in his memoir on _Die sogenannten Nulliporen_[328]; he distinguishes between _Dactyloporella_ and _Gyroporella_ by the existence of cavities in the calcareous wall of the tube in the former genus, and by their absence in the latter. The oval cavities in a _Dactyloporella_ were originally occupied by sporangia; in _Diplopora_ and _Gyroporella_ the sporangia were probably borne externally and on an uncalcified portion of the thallus.

• • • • •

In addition to the few examples of fossil species described above there are numerous others of considerable interest, which illustrate the great wealth of form among the Tertiary and other representatives of the Verticillate Siphoneae.

Reference has already been made to _Vermiporella_ as an example of a Silurian genus. Other genera have been described by Stolley from Silurian boulders in the North-German drift under the names _Palaeoporella_, _Dasyporella_ and _Rhabdoporella_[329]; the latter genus is compared with the Triassic _Diplopora_, and the two preceding with the recent _Bornetella_.

Schlüter has transferred a supposed Devonian Foraminiferal genus, _Coelotrochium_[330], to the list of Palaeozoic Siphoneae. Munier-Chalmas regards some of the fossils described by Saporta under the name of _Goniolina_[331], and classed among the inflorescences of pro-angiospermous plants, as examples of Jurassic Siphoneae. The shape and surface-features of some of the examples of _Goniolina_ suggest a comparison with Echinoid spines, but the resemblance which many of the forms in the Sorbonne collection present to large calcareous Siphoneae is still more striking. A comparison of Saporta’s fig. 5, Pl. xxxiii. and fig. 4, Pl. xxxii. in volume iv. of the _Flore Jurassique_, with the figures given by Solms-Laubach[332] and Cramer[333] of species of _Bornetella_ brings out a close similarity between _Goniolina_ and recent algae; the chief difference being the greater size of the fossil forms. The possibility of confounding Echinoid spines with calcareous Siphoneae is illustrated by Rothpletz[334], who has expressed the opinion that Gümbel’s _Haploporella fasciculata_ is not an alga but the spine of a sea-urchin.

Among Cretaceous forms, in addition to _Goniolina_, which passes upwards from Jurassic rocks, _Triploporella_[335] and other genera have been recorded.

_Uteria_[336] is an interesting type of Tertiary genera; it occurs in the form of barrel-shaped rings, which are probably the detached segments of a form in which the central axial cell was encrusted with carbonate of lime, but the sporangia and the whorls of branches differed from those of _Cymopolia_ in being without a calcareous investment.

_b._ CONFERVOIDEAE.

Without attempting to describe at length the fossil forms referred to this division of the Chlorophyceae, there is one fossil which deserves a passing notice. Brongniart in 1828[337] instituted the generic term _Confervites_ for filamentous fossils resembling recent species of confervoid algae. Numerous fossils have been referred to this genus by different authors, but they are for the most part valueless and need not be further considered. In 1887 Bornemann described some new forms which he referred to this genus from the Cambrian rocks of Sardinia. He describes the red marble of San Pietra, near Masne, as being in places full of the delicate remains of algae having the form of branched filaments, and appearing in sections of the rock as white lines on a dark crystalline matrix. In fig. 35, G, one of these Sardinian specimens is represented. This form is named _Confervites Chantransioides_[338]; the thallus consists of branched cell-filaments, having a breadth of 6–7µ, and composed of ovate cells. It is possible that this is a fragment of a Cambrian alga, but the figures and descriptions do not afford by any means convincing evidence. From post-Tertiary beds various genera, such as _Vaucheria_ and others, have been recorded, but they possess but little botanical value.

C. INCERTAE SEDIS.

_Fossils in Boghead ‘Coal’ referred by some authors to
the Chlorophyceae._

During the last few years much has been written by two French authors, Dr Renault and Prof. Bertrand, on the subject of the so-called Boghead of France, Scotland, and other countries. They hold the view that the formation of the extensive beds of this carbonaceous material was due to the accumulation and preservation of enormous numbers of minute algae which lived in Permo-Carboniferous lakes.