GEOLOGICAL DISTRIBUTION OF SHELLS.
Geological Distribution of the Bivalve and Univalve Mollusca.—If the more rare and splendid organic remains may be regarded as the "Medals of Creation," the fossil testaceous mollusca, from their durability, numbers and variety, may be considered as the current coin of Geology. Occurring in the most ancient fossiliferous strata in small numbers, and of peculiar types,—becoming more abundant and varied in the secondary formations,—and increasing prodigiously, both numerically and specifically, in the tertiary, these relics are of inestimable value in the identification of a stratum in distant regions, and in the determination of the relative age of a series of deposits. To the solution of the former problem the sagacity of the late Dr. William Smith first suggested their applicability;[393] while the idea, so happily conceived, and so philosophically candied out, by Sir C. Lyell, of arranging that heretofore chaotic mass of deposits, termed the Tertiary, into groups, by the relative number of recent and extinct species of shells, demonstrated the important aid to be derived from this class of organic remains, in the determination of some of the most difficult questions in geological science.
[393] See an interesting memoir of Dr, Smith, from the pen of his distinguished nephew, Professor Phillips.
Many useful tables have been constructed by Professor Phillips,[394] Sir C. Lyell, M. Deshayes, M. D'Orbigny, Prof. E. Forbes, and other eminent observers, to illustrate the geological distribution, in the several formations, of the genera and species of fossil shells hitherto described. To the English student, Mr. Morris's "Catalogue of British Fossils," of which an enlarged edition is in the press, will be the most valuable for reference. In the works which we have especially recommended for reference (ante, [p. 10].), figures are given of some of the characteristic shells from each formation, as follow; commencing with the most ancient deposits.
[394] A Treatise on Geology; and Art. Geology, Encyclopæd. Metropolitana.
Silurian System. Ly. p. 350.
Orthis orbicularis; Ly. fig. 409.
——— grandis;—fig. 427.
Terebratula navicula;—fig. 410.
————– Wilsoni;—fig. 413.
Pentamerus Knightii;—fig, 411.
————— lævis;—fig. 426.
Atrypa reticularis; fig. 414. Wond. p. 786.
Lingula Lewisii; Ly. fig. 412.
Strophomena depressa;—fig. 421.
Devonian System. Ly. p. 342.
Calceola sandalina; Ly. fig. 403.
Stringocephalus Burtini;—fig. 404.
Megalodon cucullatus;—fig. 405.