The Student’s Elements of Geology

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Author: Charles Lyell

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. Derbyshire, 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 constitutes 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 shells 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. Derbyshire, Ireland, Russia, etc.)

(FIGURE 485. Aviculopecten sublobatus, Phill. Mountain Limestone. Derbyshire, Yorkshire.)

(FIGURE 486. Pleurotomaria carinata, Sowerby. (P. flammigera, Phillips). Mountain Limestone. Derbyshire, 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 shell. 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 shell of this period. In the interior it is divided into chambers (Figure 487, d), the septa or partitions not being perforated as in foraminiferous shells, 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 partitions off in like manner as it advances in age a part of its spire, forming a shelly septum.

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

More than twenty species of the genus Bellerophon (see Figure 488), a shell 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 Glass-Shell, 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. Goniatites 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 shell, like a Nautilus uncoiled and straightened (Figure 489). Some species of this genus are several feet long. The Goniatite 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 Goniatite 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.

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Chicago: Charles Lyell, "Mollusca.," The Student’s Elements of Geology, ed. Bryant Conant, James and trans. Babington, B. G. (Benjamin Guy), 1794-1866 in The Student’s Elements of Geology Original Sources, accessed October 1, 2022, http://www.originalsources.com/Document.aspx?DocID=LJDB2AQCTGLNTKX.

MLA: Lyell, Charles. "Mollusca." The Student’s Elements of Geology, edited by Bryant Conant, James, and translated by Babington, B. G. (Benjamin Guy), 1794-1866, in The Student’s Elements of Geology, Original Sources. 1 Oct. 2022. http://www.originalsources.com/Document.aspx?DocID=LJDB2AQCTGLNTKX.

Harvard: Lyell, C, 'Mollusca.' in The Student’s Elements of Geology, ed. and trans. . cited in , The Student’s Elements of Geology. Original Sources, retrieved 1 October 2022, from http://www.originalsources.com/Document.aspx?DocID=LJDB2AQCTGLNTKX.