I.—Eocene. Ly. p. 174; Wond. p. 226.
II.—Miocene. Ly. p. 168.
III.—Pliocene. Ly. p. 161.
The specimens figured by Sir C. Lyell have been so carefully selected, and are so well engraven, as to present a coup-d'œil of the most characteristic shells of the three grand divisions of the Tertiary Deposits.
I have reserved for especial mention in this place, the work, which will afford the student of British fossil Conchology the most important aid in the identification of specimens, namely, the "Mineral Conchology of Great Britain," by the late eminent naturalist, Mr. James Sowerby, and continued by his son, Mr. James De Carle Sowerby; in six volumes 8vo., with several hundred coloured plates. Unfortunately, this work has long been discontinued; and the rapid progress of discovery, and the numerous foreign publications on every department of fossil conchology, almost forbid the hope that it will be resumed by the present proprietor. Although the high price of this work places it beyond the reach of many individuals, it will be found in most libraries of Natural History.
ON COLLECTING FOSSIL SHELLS.
On the Collection and Arrangement of Fossil Shells.—The instructions already given for the collection of corals, echinoderms, &c., will have familiarized the student with the methods generally adopted, and render it unnecessary to enter into much detail. The shells in arenaceous deposits, particularly in those of the Tertiary formations, are commonly so perfect, as merely to require careful removal: those in the clays are more fragile, and must be extracted with great caution; and, when very delicate, should be left attached to the clay or shale. The specimens extracted entire may be kept either in paper trays, lined with wadding, or fixed to pieces of card or thin board covered with paper, by thick gum-water; three or four specimens being attached in different positions, so as to expose the essential characters, as the aperture, spire, and back of the univalves, and the hinge, muscular imprints, &c. of the bivalves. Where only casts remain, search should be made for an impression of the outer surface of the shell, and a cast taken of it in wax, kneaded bread, or plaster of Paris. In indurated clays, sometimes both shells and casts may be obtained; and a specimen of each should be preserved. Mastic varnish, or solution of gum tragacanth, delicately applied to fragile shells, tends to preserve them, and improves their appearance. It is desirable to collect the same species in various states of growth; the form of the young shell (as in Rostellaria ampla, of Solander) often differing essentially from that of the adult. It will be found convenient to have trays or boards of different colours; and to select one tint for the shells collected from a particular formation, or deposit; for example, the newer Tertiary may be placed on yellow paper; the older, or Eocene fossils, on light-blue. It is also desirable to separate the marine from the fresh-water species.
Shells imbedded in chalk, limestone, &c., often require much labour to display their more delicate and important characters. For clearing chalk specimens, a stout penknife, and a few gravers or gouges of various sizes, will be necessary; and by a little practice, the spines of the Spondylus ([Lign. 128]), and the beaks and hinge of Inocerami ([Lign. 129]), &c., may be readily exposed. A small stiffish brush, used with water, is also serviceable. The shells in compact stone, as those of the mountain limestone, must generally be cleared with the hammer and chisel. Common species may be broken out, and, from several examples, probably one or two will be found perfect; but choice and rare specimens should not be thus risked; they will amply repay the trouble of the less expeditious method of chiselling away the surrounding stone. Casts may be taken in gutta percha, &c.
To determine the names of the specimens that he has collected should be the next care of the student. No method will so readily initiate the young collector in fossil conchology, as the careful examination of a small series of the common species, with their names attached.[396] By the geological map,[397] the nature of the deposit in which the locality of the specimens is situated, may be ascertained; and the remarks previously advanced on the prevailing shells of each formation, will afford a general idea of the genera to which they belong; and, by referring to the figures quoted, the specific names may be determined.
[396] Such a series may be obtained, at very little cost, of dealers in objects of natural history; as, Messrs. Tennant, Sowerby, the British Natural History Society, &c. See Appendix.