32 



MANTJAL OP THE MOLLTJSCA. 



In many bivalve shells there occurs a minute tubular struc- 

 ture, which is very conspicuous in some sections of pinna and 

 oyster-shell. This tubular structure is frequently occasioned 

 by the growth of a confervoid sponge, hence great care is 

 required in determining whether the perforations are an 

 essential part of the shell. 



The hrachiopoda exhibit a characteristic structure by which 

 the smallest fragment of their shells may be determined ; it 

 consists of elongated and curved cells matted together, and 

 often perforated by circular holes, arranged in quincunx order 

 (Fig. 22). 



But the most complex shell-structure is presented by the 

 porcellanous gasteropoda. These consist of three strata which 

 readily separate in fossil shells, on account of the removal of 



their animal cement. In Fig. 

 24, a represents the outer, h 

 the middle, and c the inner 

 stratum ; they may be seen 

 also in Fig. 25. Each of 

 these three strata is com- 

 posed of very numerous ver- 

 tical plates, like cards placed 

 on edge ; and the direction 

 of the plates is sometimes 

 transverse in the central 

 stratum, and lengthwise in the outer and inner (as in cyprcea, 

 cassis, ampullaria, and hulimus), or longitudinal in the middle 

 layer and transverse in the others (e. g. comis, pyrula, oliva, 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. — [Bowerhanh.)^ 



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

 peculiar structure ; and the Hippurite is distinguished by a 



Fig. 24. Sections of a Cone.* 



* Sections of Conus ponderosus, Bmg., from the Miocene of the Touraine. A, 

 longitudinal section of a fragment ; B, complete horizontal section ; a, outer layer ; 6, 

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



t It is necessary to bear in mind that fossil shells are often pseudomorphous, 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 particlePi 

 like the rocks in which they occur. 



