Imágenes de página
PDF

SECTION VIII.

FORAMINATED POLYTHALAMOUS SHELLS.

Nummulites.

If the present were a fit occasion for such minute inquiries, the investigations of the various known species of Microscopic shells would unfold a series of contrivances, having relation to the economy of the minute Cephalopods by which they were constructed, not less striking than those we have been examining in the shells of extinct Genera and species of larger Cephalopods. M. D'Orbigny has noticed from 600 to 700 species of these shells, and has prepared magnified models of 100 species, comprehending

all the Genera.*

The greater number of these shells are microscopic, and swarm in the Mediterranean and Adriatic. Their fossil species abound chiefly in the Tertiary formations, and have hitherto been noticed principally in Italy. (See Soldani, as quoted at page 97 of this volume.) They occur also in the Chalk of Meudon, in the Jura Limestone of the Charente inferieure, and the Oolite of Calne. They have been found by the Marquis of Northampton in Chalk flints from the neighbourhood of Brighton.

* M. D'Orbigny, in his classification of the shells of Cephalopodous Mollusks, has established three orders. 1. Those that have but a single chamber, like the shell of the sepia and horny pen of the Loligo. 2. Polythalamous shells, which have a siphuncle passing through all the internal chambers, and which terminate in a large external chamber, beyond the last partition, such as Nautili, Ammonites, and Belemnites. 3. Poly thalamous internal shells, which have no chamber beyond their last partition.

Shells of this order have no siphuncle, but the chambers communicate with each other by means of one or many small foramina. On this distinction he has founded his Order Foraminiferes, containing five families and

fifty-two genera. It may be necessary to apprize the reader that doubts have been enter.

tained as to the cephalopodous structure of some of these minute multilocu

lar shells; and that there are not wanting those who attribute to them a

different organization.

\

The Nummulite is the only Genus I shall select on the present occasion from this Order. It is included in M. D'Orbigny's Section Nautiloids.

Nummulites (PI. 44, Fig. 6, 7,) are so called from their resemblance to a piece of money, they vary in size from that of a crown piece to microscopic littleness; and occupy an important place in the history of fossil shells, on account of the prodigious extent to which they are accumulated in the later members of the Secondary, and in many of the Tertiary strata. They are often piled on each other nearly in as close contact as the grains in a heap of corn. In this state they form a considerable portion of the entire bulk of many extensive mountains, e. g. in the Tertiary limestones of Verona and Monte Bolca, and in secondary strata of the Cretaceous formation in the Alps, Carpathians, and Pyrenees. Some of the pyramids and the Sphinx, of Egypt, are composed of limestone loaded with Nummulites.

It is impossible to see such mountain-masses of the remains of a single family of shells thus added to the solid materials of the globe, without recollecting that each individual shell once held an important place within the body of . a living animal; and thus recalling our imagination to those distant epochs when the waters of the ocean which then covered Europe were filled with floating swarms of these extinct Mollusks, thick as the countless myriads of Bero-e and Clio Borealis that now crowd the waters of the polar seas.* ,

• We have an analogy to this supposed state of crowded population of Nummulites in the ancient sea, in the marvellous fecundity of the northern ocean at the present time. It is stated by Cuvier, in his memoir on the

Vol. i.—25

[graphic]

The Nummulites, like the Nautilus and Ammonite, are divided into air-chambers, which served the office of a float: but there is no enlargement of the last chamber which could have contained any part of the body of the animal. The chambers are very numerous, and minutely divided by transverse plates; but are without a siphuncle.* The form of the essential parts varies in each species of this genus, but their principles of construction, and manner of operation, appear in all to have been the same.

The remains of the Nummulites are not only animal bodies which have contributed to form the calcareous strata of the crust of the earth; other, and more minute species of Chambered shells have also produced great, and most surprising effects. Lamarck (Note, v. 7. p. 611,) speaking of the Miliola, a small moltilocular shell, no larger than a millet seed, with which the strata of many quarries in the neighbourhood of Paris are largely interspersed, notices the important influence which these minute bodies have exercised by reason of their numerical abundance. We scarcely condescend, says he, to examine microscopic shells, from their insignificant size: but we cease to think them insignificant, when we reflect that it is by means of the smallest objects, that Nature every where produces her most remarkable and astonishing phenomena. Whatever she may seem to lose in point of volume in the production of living bodies, is amply made up by the number of the individuals, which she multiplies with admirable promptitude to infinity. The remains of such minute animals have added much more to the mass of materials which compose the exterior crust of the globe, than the bones of Elephants, Hippopotami, and Whales.

Clio Borealis, that in calm weather, the surface of the water in these seas swarms with such millions of these mollusks (rising for a moment to the air . at the surface, and again instantly sinking towards the bottom,) that the whales can scarce open their enormous mouths without gulping in thousands of these little gelatinous creatures, an inch long, which, together with Medusae, and some smaller animals, constitute the chief articles of their food; and we have a farther analogy in the fact mentioned in Jameson's Journal, vol. ii. p. 12. "That the number of small Medusae in some parts of the Greenland seas is so great, that in a cubic inch, taken up at random, there are no less than 64. In a cubic foot this will amount to 110,592; and in a cubic mile (and there can be no doubt of the water being charged with them to that extent,) the number is such, that allowing one person to count a million in a week, it would have required 80,000 persons, from the creation of the world, to complete the enumeration."—See Dr. Kidd's admirable Introductory Lecture to a course of Comparative Anatomy, Oxford, 1624, p. 35.

* In PI. 44, Figs. 6, 7, sections of two species of Nummulite are copied from Parkinson. These show the manner in which the whorls are coiled up round each other, and divided by oblique septa.

CHARTER XVI.

Proofs of Design in the Structure of Fossil Articulated, Animals.

The third grand division in Cuvier's arrangement of the animal kingdom, viz. the articulated animals, comprehends four classes.

1. The Annelidans, or worms with red blood.

2. Crustaceans, most familiar to us under the form of Crabs and Lobsters.

3. Arachnidans, or Spiders.

4. Insects.

SECTION I.

First Class of Articulated Animals.

f' -

FOSSIL ANNELIDANS.

However numerous may have been the ancient species of Annelidans without a shelly covering, naked worms of this class can have left but slight traces of their existence, except the holes they perforated, and the little accumulations of sand or mud cast up at the orifice of these perforations; in a preceding chapter* we have noticed examples of this kind. We have also abundant evidence of the early and continued prevalence of that order of Annelidans, which formed shelly calcareous tubes, in the occurrence of fossil Serpute in nearly all formations, from the Transition periods to the present time.

SECTION II.
Second Class of Articulated Animals.

FOSSIL CRUSTACEANS.

The history of fossil Crustaceans has been hitherto almost untouched by Palaeontologists, and their relations to the existing Genera of this great Class of the Animal Kingdom are to olittle known to admit of discussion in this place.

See note at pages 198—199.

« AnteriorContinuar »