[Y] See 'Wonders of Geology,' 6th Edition, p. 402.
The cretaceous strata contain many hundred species of bivalves and univalves, by far the greater part of which belong to extinct genera; and the species, with but four or five exceptions, are unknown in more recent deposits. In loose sandy strata, fossil shells are oftentimes beautifully preserved, and may be obtained in as perfect a condition as if gathered from the sands on the sea-shores: such is the state of the specimens which abound in the sandy clays near Barton in Hampshire, and in the "Crag" of Essex and Suffolk. In certain beds of clay, shells are also found entire; sometimes retaining the epidermis, and the cartilaginous ligament of the hinge. The bivalves in the white chalk are generally perfect; but the univalves, probably from the more delicate structure of the originals, seldom retain any vestiges of the shell, excepting portions of the internal nacreous coat adhering to the chalk casts, which have been moulded in the interior of the shells.
Lign. 19:—Bivalve shells (Terebratulæ) from Chalk (natural size).
1, 2. Plicated species. 1. T. octoplicata. 2. T. subplicata.
3, 4. Smooth species. 3. T. semiglobosa. 4. T. subrotunda.
TEREBRATULÆ FROM CHALK.
In some of the cretaceous strata several extinct species of Oyster, Scallop, Arca, Tellina, and other well-known marine bivalves abound; and with them are associated many genera of which no living species have been observed. Among the bivalves that prevail in the English chalk, are three or four kinds of Terebratulæ: which are small, elegant, subglobular shells, belonging to a family of which nearly 500 species, referable to several genera, have been obtained from the British strata.[Z] Certain genera are restricted to the most ancient sedimentary rocks, in which they occur in almost incredible numbers; others have a wider range and are met with in the later secondary deposits; while a few are found in the newest beds, and have living representative species in the seas of warm climates. From the immense antiquity of their lineage, these Terebratulæ have been humourously termed the "fossil aristocracy." Some of the most common chalk species are figured of the natural size in [Lign. 19]. When living the animal was attached to a rock or other body by means of a byssus or peduncle, exserted through the aperture in the beak or curved extremity of the largest valve.[AA] The shells of the smooth Terebratulæ are full of minute holes or perforations, which may readily be distinguished with a lens of moderate power.
[Z] See 'Wonders of Geology,' 6th Edit. p. 329.
[AA] In the Conchological Gallery of the British Museum there is a group of thirty or forty recent Terebratulæ attached to a stone by their peduncles; from Australia.