384 THE MEDALS OF CREATION. Cuap. XI.’ 
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 mate- 
rially differ from those on the interior, and, consequently, 
the cagt, 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, there-— 
fore, in such examples, so little resembles the shell, that an_ 
inexperienced collector may readily suppose it belongs to a_ 
different. species. The bivalve called Zrigonia, Lign. 127, _ 
figs. 1, 2, is an instance of this contrast. : 
The peal slab of the Septarium, Lign. 122, jig. 2, - 
demonstrates another condition of fossil shells—that of a 4 
compact argillaceous limestone—and entire beds of marble 3 
are aed of an aggregation of this kind, formed of shells © 
and other animal exuvie, consolidated by mineral a 
tions. In the older secondary strata this state prevails ; and — 
the beautiful markings of many valuable marbles, are merely i 
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 consannie 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. 4 
We have here a solid mass of stone, composed of several 7 
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 been obtained at different 
soundings along this part of the Sussex coast ; and in some — 
specimens numerous other species of recent shells, as oysters, — 
* 
a 
Mey 
+E. 
3. 
