LIGN. 86. TURRITELLE, FROM BRACKLESHAM, SUSSEX.
(Drawn by Mr. Joseph Dinkel.)

Fig. 1.-TURRITELLA CONOIDEA; the perfect shell: nat.
2.-SEPTARIUM, with TURKITELLE; a polished
slab: nat.

3

Tert.

exterior of the shell do not materially differ from those on the interior, and, consequently, the cast, fig. 3, and the shell, fig. 1, resemble each other; but in many species there is a striking contrast between the outer and inner surfaces, the external aspect being strongly ornamented, while the internal is smooth; the cast, therefore, in such examples, so little resembles the shell, that an inexperienced collector may readily suppose it belongs to a different species. The bivalve called Trigonia, Lign. 91, figs. 1, 2, is an instance of this contrast.

The polished slab of the Septarium, Lign. 86, fig. 2, demonstrates another condition of fossil shells-that of a compact argillaceous limestone-and entire beds of marble are composed of an aggregation of this kind, formed of shells and other animal exuviæ, consolidated by mineral infiltrations. In the older secondary strata this state prevails; and the beautiful markings of many valuable marbles, are caused by sections of the enclosed shells. But this process is not restricted to the deposits of ancient date; at the present moment the same operation is silently but constantly going on in our seas, and an examination of the specimen, Lign. 87, will afford an exemplification of the manner in which these shelly limestones are produced.

We have here a solid mass of stone, composed of several recent species of shells, corals, &c. It is a fragment of a large block, dredged up from the British Channel, off Brighton. Similar masses have

SHELL-CONGLOMERATE; now forming in the British
Channel. Dredged off Brighton.

(Drawn by Mr. Joseph Dinkel.)

Fig. 1.-An Aggregation of Shells and Corals; the interstices are filled up with sand, and the mass is consolidated by an infiltration of carbonate of lime.

2.-TROCHUS ZIZI PHINUS; extricated from the mass with the following.

5.-Portion of a FLUSTRA; magnified.

6.-SABELLA.

THE MEDALS OF CREATION.

CHAP. XII.

been obtained at different soundings along this part of the Sussex coast; and in some specimens numerous other species of recent shells, as oysters, mussels, whelks, &c. enter into the composition of the consolidated rock. The shelly and coralline limestones and sandstones, so abundant in the ancient strata of England, have been formed in a similar manner; and when the modern conglomerate of Brighton shall have been permeated with crys talline matter, and subjected to great pressure by superincumbent deposits, through countless centuries, and at length be elevated above the waters, it will constitute beds of shell-marble, in some mountain range, and become an interesting, perhaps the only memento, of the races of mollusca and polypiaria of the present seas, when all record and traces of Great Britain and its inhabitants shall be destroyed.

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. 88.); 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

(Drawn by Mr. Joseph Dinkel.)

Fig. 1.--A mass of Cockle-shells and Whelks, consolidated into a coarse limestone.

2, 4.-One of the shells, CARDIUM EDULE, extracted from the

block.

3.-A slice of the same, polished, the markings on the surface being derived from sections of the shells.