The Student's Elements of Geology Part 52

b. Remains of the spathe magnified.

c. Portion of spike magnified.

d. One of the calyces magnified.)

In the coal-measures of Granton, near Edinburgh, a remarkable fossil (Figure 473) was found and described in 1840, by Dr. Robert Paterson. (Transactions of the Botanical Society of Edinburgh volume 1 1844.) It was compressed between layers of bituminous shale, and consists of a stem bearing a cylindrical spike, a, which in the portion preserved in the slate exhibits two subdivisions and part of a third. The spike is covered on the exposed surface with the four-cleft calyces of the flowers arranged in parallel rows. The stem shows, at b, a little below the spike, remains of a lateral appendage, which is supposed to indicate the beginning of the spathe. The fossil has been referred to the Aroidiae, and there is every probability that it is a true member of this order. There can at least be no doubt as to the high grade of its organisation, and that it belongs to the monocotyledonous angiosperms. Mr. Carruthers has carefully examined the original specimen in the Botanical Museum, Edinburgh, and thinks it may have been an epiphyte.

CLIMATE OF THE COAL PERIOD.

As to the climate of the Coal, the Ferns and the Coniferae are perhaps the two cla.s.ses of plants which may be most relied upon as leading us to safe conclusions, as the genera are nearly allied to living types. All botanists admit that the abundance of ferns implies a moist atmosphere. But the coniferae, says Hooker, are of more doubtful import, as they are found in hot and dry, and in cold and dry climates; in hot and moist, and in cold and moist regions. In New Zealand the coniferae attain their maximum in numbers, const.i.tuting 1/62 part of all the flowering plants; whereas in a wide district around the Cape of Good Hope they do not form 1/1600 of the phenogamic flora. Besides the conifers, many species of ferns flourish in New Zealand, some of them arborescent, together with many lycopodiums; so that a forest in that country may make a nearer approach to the carboniferous vegetation than any other now existing on the globe.

MARINE FAUNA OF THE CARBONIFEROUS PERIOD.

It has already been stated that the Carboniferous or Mountain Limestone underlies the coal-measures in the South of England and Wales, whereas in the North, and in Scotland, marine calcareous rocks partly of the age of the Mountain Limestone alternate with shales and sandstones, containing seams of coal. In its most calcareous form the Mountain Limestone is dest.i.tute of land- plants, and is loaded with marine remains-- the greater part, indeed, of the rock being made up bodily of crinoids, corals, and bryozoa with interspersed mollusca.

CORALS.

(FIGURE 474. Palaeozoic type of lamelliferous cup-shaped Coral. Order ZOANTHARIA RUGOSA, Milne Edwards and Jules Haime.

a. Vertical section of Campophyllum flexuosum, (Cyathophyllum, Goldfuss); 1/2 natural size: from the Devonian of the Eifel. The lamellae are seen around the inside of the cup; the walls consist of cellular tissue; and large transverse plates, called tubulae, divide the interior into chambers.

b. Arrangement of the lamellae in Polycoelia profunda, Germar, sp.; natural size: from the Magnesian Limestone, Durham.

This diagram shows the quadripart.i.te arrangement of the primary septa, characteristic of palaeozoic corals, there being four princ.i.p.al and eight intermediate lamellae, the whole number in this type being always a multiple of four.

c. Stauria astraeiformis, Milne Edwards. Young group, natural size. Upper Silurian, Gothland. The lamellae or septal system in each cup are divided by four prominent ridges into four groups.)

(FIGURE 475. Neozoic type of lamelliferous cup-shaped Coral. Order ZOANTHARIA APOROSA, M. Edwards and J. Haime.

a. Parasmilia centralis, Mantell, sp. Vertical section; natural size. Upper Chalk, Gravesend. In this type the lamellae are ma.s.sive, and extend to the axis or columella composed of loose cellular tissue, without any transverse plates like those in Figure 474, a.

b. Cyathina Bowerbankii, Ed. and H. Transverse section, enlarged. Gault, Folkestone. In this coral the primary septa are a multiple of six. The twelve princ.i.p.al plates reach the columella, and between each pair there are three secondaries, in all forty-eight. The short intermediate plates which proceed from the columella are not counted. They are called pali.

c. Fungia patellaris, Lamarck. Recent; very young state. Diagram of its six primary and six secondary septa, magnified. The s.e.xtuple arrangement is always more manifest in the young than in the adult state.)

The corals deserve especial notice, as the cup-and-star corals, which have the most ma.s.sive and stony skeletons, display peculiarities of structure by which they may be distinguished generally, as MM. Milne Edwards and Haime first pointed out, from all species found in strata newer than the Permian. There is, in short, an ancient or PALAEOZOIC, and a modern or NEOZOIC type, if, by the latter term, we designate (as proposed by Professor E. Forbes) all strata from the tria.s.sic to the most modern, inclusive. The accompanying diagrams (Figures 474, 475) may ill.u.s.trate these types.

It will be seen that the more ancient corals have what is called a quadripart.i.te arrangement of the chief plates or LAMELLAE-- parts of the skeleton which support the organs of reproduction. The number of these lamellae in the Palaeozoic type is 4, 8, 16, etc.; while in the Neozoic type the number is 6, 12, 24, or some other multiple of six; and this holds good, whether they be simple forms, as in Figures 474, a, and 475, a, or aggregate cl.u.s.ters of corallites, as in 474, c. But further investigations have shown in this, as in all similar grand generalisations in natural history, that there are exceptions to the rule. Thus in the Lower Greensand Holocystis elegans (Ed. and H.) and other forms have the Palaeozoic type, and Dr. Duncan has shown to what extent the Neozoic forms penetrate downward into the Carboniferous and Devonian rocks.

(FIGURE 476. Lithostrotion basaltiforme, Phil. sp. (Lithostrotion striatum, Fleming; Astraea basaltiformis, Conyb. and Phill.). England, Ireland, Russia, Iowa, and westward of the Mississippi, United States. (D.D. Owen.)

(FIGURE 477. Lonsdaleia floriformis, Martin, sp., M. Edwards. (Lithostrotion floriforme, Fleming. Strombodes.) a. Young specimen, with buds or corallites on the disk, ill.u.s.trating calicular gemmation.

b. Part of a full-grown compound ma.s.s. Bristol, etc.; Russia.)

From a great number of lamelliferous corals met with in the Mountain Limestone, two species (Figures 476, 477) have been selected, as having a very wide range, extending from the eastern borders of Russia to the British Isles, and being found almost everywhere in each country. These fossils, together with numerous species of Zaphrentis, Amplexus, Cyathophyllum, Clisiophyllum, Syringopora, and Michelinia, form a group of rugose corals widely different from any that followed them. (For figures of these corals, see Palaeontographical Society's Monographs 1852.)

BRYOZOA AND CRINOIDEA.

(FIGURE 478. Cyathocrinus pla.n.u.s, Miller. Body and arms. Mountain Limestone.)

(FIGURE 479. Cyathocrinus caryocrinoides, M'Coy.

a. Surface of one of the joints of the stem.

b. Pelvis or body; called also calyx or cup.

c. One of the pelvic plates.)

Of the Bryozoa, the prevailing forms are Fenestella, Hemitrypa, and Polypora, and these often form considerable beds. Their net-like fronds are easily recognised. Crinoidea are also numerous in the Mountain Limestone (see Figures 478, 479), two genera, Pentremites and Codonaster, being peculiar to this formation in Europe and North America.

(FIGURE 480. Palaechinus gigas, M'Coy. Reduced one-third. Mountain Limestone.

Ireland.)

In the greater part of them, the cup or pelvis, Figure 479, b, is greatly developed in size in proportion to the arms, although this is not the case in Figure 478. The genera Poteriocrinus, Cyathocrinus, Pentremites, Actinocrinus, and Platycrinus, are all of them characteristic of this formation. Other Echinoderms are rare, a few Sea-Urchins only being known: these have a complex structure, with many more plates on their surface than are seen in the modern genera of the same group. One genus, the Palaechinus (Figure 480), is the a.n.a.logue of the modern Echinus, but has four, five, or six rows of plates in the interambulacral region or area, whereas the modern genera have only two. The other, Archaeocidaris, represents, in like manner, the Cidaris of the present seas.

MOLLUSCA.

(FIGURE 481. Productus semireticulatus, Martin, sp. (P. antiquatus, Sowerby.) Mountain Limestone. England, Russia, the Andes, etc.)

(FIGURE 482. Spirifera trigonalis, Martin, sp. Mountain Limestone. Derbys.h.i.+re, etc.)

(FIGURE 483. Spirifera glabra, Martin, sp. Mountain Limestone.)

The British Carboniferous mollusca enumerated by Mr. Etheridge comprise 653 species referable to 86 genera, occurring chiefly in the Mountain Limestone.

(Quarterly Geological Journal volume 23 page 674 1867.) Of this large number only 40 species are common to the underlying Devonian rocks, 9 of them being Cephalopods, 7 Gasteropods, and the rest bivalves, chiefly Brachiopoda (or Palliobranchiates). This latter group const.i.tutes the larger part of the Carboniferous Mollusca, 157 species being known in Great Britain alone, and it will be found to increase in importance in the fauna of the primary rocks the lower we descend in the series. Perhaps the most characteristic sh.e.l.ls of the formation are large species of Productus, such as P. giganteus, p.

hemisphericus, P. semireticulatus (Figure 481), and P. scabriculus. Large plaited spirifers, as Spirifera striata, S. rotundata, and S. trigonalis (Figure 482), also abound; and smooth species, such as Spirifera glabra (Figure 483), with its numerous varieties.

(FIGURE 484. Terebratula hastata, Sowerby, with radiating bands of colour.

Mountain Limestone. Derbys.h.i.+re, Ireland, Russia, etc.)

(FIGURE 485. Aviculopecten sublobatus, Phill. Mountain Limestone. Derbys.h.i.+re, Yorks.h.i.+re.)

(FIGURE 486. Pleurotomaria carinata, Sowerby. (P. flammigera, Phillips).

Mountain Limestone. Derbys.h.i.+re, etc.)

Among the brachiopoda, Terebratula hastata (Figure 484) deserves mention, not only for its wide range, but because it often retains the pattern of the original coloured stripes which ornamented the living sh.e.l.l. These coloured bands are also preserved in several lamellibranchiate bivalves, as in Aviculopecten (Figure 485), in which dark stripes alternate with a light ground.

In some also of the spiral univalves the pattern of the original painting is distinctly retained, as in Pleurotomaria (Figure 486), which displays wavy blotches, resembling the colouring in many recent trochidae.

(FIGURE 487. Euomphalus pentangulatus, Sowerby. Mountain Limestone.

a. Upper side.

b. Lower or umbilical side.

c. View showing mouth, which is less pentagonal in older individuals.

d. View of polished section, showing internal chambers.)

Some few of the carboniferous mollusca, such as Avicula, Nucula (sub-genus Ctenodonta), Solemya, and Lithodomus, belong no doubt to existing genera; but the majority, though often referred to as living types, such as Isocardia, Turritella, and Buccinum, belong really to forms which appear to have become extinct at the close of the Palaeozoic epoch. Euomphalus is a characteristic univalve sh.e.l.l of this period. In the interior it is divided into chambers (Figure 487, d), the septa or part.i.tions not being perforated as in foraminiferous sh.e.l.ls, or in those having siphuncles, like the Nautilus. The animal appears to have retreated at different periods of its growth from the internal cavity previously formed, and to have closed all communication with it by a septum. The number of chambers is irregular, and they are generally wanting in the innermost whorl. The animal of the recent Turritella communis part.i.tions off in like manner as it advances in age a part of its spire, forming a sh.e.l.ly septum.

(FIGURE 488. Bellerophon costatus, Sowerby. Mountain Limestone.)

More than twenty species of the genus Bellerophon (see Figure 488), a sh.e.l.l like the living Argonaut without chambers, occur in the Mountain Limestone. The genus is not met with in strata of later date. It is most generally regarded as belonging to the pelagic Nucleobranchiata and the family Atlantidae, partly allied to the Gla.s.s-Sh.e.l.l, Carinaria; but by some few it is thought to be a simple form of Cephalopod.

(FIGURE 489. Portion of Orthoceras laterale. Phill. Mountain Limestone.)

(FIGURE 490. Goniat.i.tes crenistra, Phillips. Mountain Limestone. North America, Britain, Germany, etc.

a. Lateral view.

b. Front view, showing the mouth.)

The carboniferous Cephalopoda do not depart so widely from the living type (the Nautilus) as do the more ancient Silurian representatives of the same order; yet they offer some remarkable forms. Among these is Orthoceras, a siphuncled and chambered sh.e.l.l, like a Nautilus uncoiled and straightened (Figure 489). Some species of this genus are several feet long. The Goniat.i.te is another genus, nearly allied to the Ammonite, from which it differs in having the lobes of the septa free from lateral denticulations, or crenatures; so that the outline of these is angular, continuous, and uninterrupted. The species represented in Figure 490 is found in most localities, and presents the zigzag character of the septal lobes in perfection. The dorsal position of the siphuncle, however, clearly distinguishes the Goniat.i.te from the Nautilus, and proves it to have belonged to the family of the Ammonites, from which, indeed, some authors do not believe it to be generically distinct.

FOSSIL FISH.

(FIGURE 491. Psammodus porosus, Aga.s.siz. Bone-bed, Mountain Limestone. Bristol, Armagh.)