MODERN SHELL-LIMESTONE.
Off the Kentish coast, near the mouth of the Thames, a bank of consolidated shells, chiefly of one species, is in the progress of formation, from which blocks may be obtained of great firmness and solidity ([Lign. 124]); these, when cut and polished (fig. 3), display a variety of markings, produced by the sections of the shells. Extensive shoals of loose shells, composed almost wholly of the Cardium edule, exist in several localities, near the embouchure of the Thames; and these are continually shifting with the changes of the wind and tide; it is only in a few places that consolidated blocks occur, like that of which a fragment is figured in [Lign. 124]. These examples of shelly limestones and sandstones now in progress of formation will familiarize the student with the nature and origin of those ancient deposits of a similar character, which contain extinct species and genera of mollusca.
"The vast deposits of fluviatile shells which exist in Florida, at Picolata, Volusia, and Enterprize are of great geological interest. The two latter places present bluffs and hills of from forty to fifty feet in height, extending half a mile or more from the river, that are composed of scarcely anything but well-preserved shells of Paludina vivipara, Ampullaria depressa, some undetermined species of Unio, Helix septemvolvis, Melania, and a few others. There is but a scanty mixture of earth, and the shells are clean, and look as if they had been washed ashore after the death of their inhabitants. In some places the beds are sandy, and are hardening into a calcareous shelly sandstone. In one such bed the superficial stratum furnished a few bones of turtles and undetermined fragments, the bones of some large vertebrate animal. This is, I believe, the locality where Count Pourtalés collected human bones in a recent sandstone.... No microscopical forms were detected in these beds after the most careful search."[352]
[352] Dr. J, W. Bailey, in Smithsonian Contributions, vol. ii. Article viii. p. 23.
Lign. 125. Terebratula and Rhynchonella; nat.
Chalk. Lewes.
| Fig. | 1.— | Rhynchonella plicatilis. |
| 1a.— | The same species, partly open. | |
| 2.— | Rhynchonella subplicata. | |
| 2a.— | Front view of the same. | |
| 3.— | Terebratula semiglobosa; side view. | |
| 3a.— | The same species, seen from above. | |
| 4.— | Terebratula subrotunda. |
Fossil Shells of the Brachiopodous Mollusca.—These are bivalve shells, of which nearly five hundred species are found in the British strata. They occur in incredible numbers in the ancient rocks, to which several genera are restricted; while some continue through all the formations, and inhabit the present seas; but the existing genera are few.