The specimen, _Lign. 122, fig. 2_, from the tertiary strata at Bracklesham Bay, Sussex, is a polished slice of indurated argillaceous limestone, from a septarium (_nodule divided by fissures_), abounding in spiral univalve shells, called _Turritellæ_. _Fig. 1_ is a perfect shell of the same species, extracted from soft clay; and _fig. 3_, a cast in calcareous spar, obtained from the septarium. In the polished slab, _fig. 2_, sections of numerous shells are seen. The dark partitions, or septa, are veins of spar, which occupy interstices that have been formed in the clay-nodule by shrinking; and if the specimen be closely examined, the shells will be found split across and displaced by the fissures; thus presenting an interesting illustration of the faults, or dislocations, of the strata, so familiar to the geological observer. In the present instance, the lines on the 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. 127, figs. 1, 2_, is an instance of this contrast. The polished slab of the Septarium, _Lign. 122, 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 merely 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. 123_, will afford an exemplification of the manner in which these shelly limestones are produced. [Sidenote: BRIGHTON SHELL-CONGLOMERATE.] [Illustration: Lign. 123. Shell-Conglomerate; now forming in the British Channel. _Dredged off Brighton._ 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 ziziphinus; extricated from the mass with the following: 3.--Pecten opercularis. 4.--Serpula. 5.--Portion of a Cellepora; magnified. 6.--Sabella. ]