40 



MANUAL or THE MOLLUSC A. 



animal cement. In fi^. 24. a represents the outer, b the middle, 

 and c the inner stratum ; they 

 may be seen, also, in fig. 25. 



Each of these three strata 

 is composed of very nume- 

 rous vertical plates, like cards 

 placed on edge ; and the di- 

 rection of the plates is some- 

 times transverse in the central 

 stratum, and lengthwise in ^^S- 24- Sections of a cone * 

 the outer and inner (as in cyprcea, cassis, ampullaria, and buli- 

 nius), or longitudinal in the middle layer, and transverse in the 

 others {e. g. comis, pyrula, oUva, and voluta). 



Each plate, too, is composed of a series of prismatic cells, 

 arranged obliquely (45'^), and their direction being changed in 

 the successive plates, they cross each other at right angles. 

 Tertiary fossils best exhibit this structure, either at their broken 

 edge, or in polished sections. f (Bowerbank). 



The argonaut-shell, and the bone of the cuttle-fish, have a 

 peculiar structure ; and the Hippurite is distinguished by a can- 

 cellated texture, unKke any other shell, except, perhaps, some of 

 the cardiacece and chamacece. 



Epidermis. AU shells have an outer coat of animal matter 

 called the " epidermis" (or periostracum), sometimes thin and 

 transparent, at others thick and opaque. It is thick and olive- 

 colom*ed in aU fresh-water shells, and in many arctic sea-sheUs 

 {e. g. cyprina and astarte) ; the colours of the land-shells often 



* Sections of conus ponder oms, Brug., from the Miocene of the Touraine. 

 A, longitudinal section of a fragment, B, complete horizontal section; a, outer 

 layer; b, middle; c, inner layer; d, e,f, lines of growth. 



t It is necessary to bear in mind that fossil shells are often pseudomor- 

 phxnts, or mere casts, in spar or chalcedony, of cavities once occupied by shells ; 

 such are the fossils found at Blackdown, and many of the London clay fossils 

 at Barton. The Palaeozoic fossils are often metamorphic, or have undergone 

 a re-arrangement of their particles, like the rocks in wliich they occur. 



