32 



MAXUAL OF THE MOLLUSCA. 



In many biyalye shells tliGre occurs a minute tulmlar struc- 

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

 oyster-slioll. This tubular structure is frequently occasioned 

 by the growth of a conferyoid sponge, hence groat care is 

 required in determining whether the perforations are an 

 essential part of the shell. 



The Iracliiopoda exhibit a characteristic structure by which 

 the smallest fragment of their shells may be determined ; it 

 consists of elongated and curyed cells matted together, and 

 often i^erforated by circular holes, arranged in quincunx order 

 (Fig. 22). 



Eut the most complex shell -structure is presented by the 

 porccllanous gasterojioda. These consist of three strata which 

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



theii^ 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 yery numerous yer- 

 tical plates, like cards placed 

 on edge ; and the direction 

 of the plates is sometimes 

 rig. 24. Sections of a Cone.* transyerse in the central 



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

 cassis, cmijniUaria, and Vidimus), or longitudinal in the middle 

 layer and transyerse in the others (e. g. conus, pyrula, oliva, and 

 voluta). 



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

 arranged obliquely {4.5'^), and their dii'ection being changed in 

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

 Tertiary fossils best exhibit this structure, cither at their broken 

 edge, or in polished sections. — {Boiuerhanlc.)\ 



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

 peculiar structure ; and the Hippurite is distinguished by a 



a b c 



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

 longitudinal section of a fragment; B, complete horizontal section ; a, outer laj-cr ; h, 

 middle ; c, inner layer; d, e,f, lines of gro-wth. 



t It is necessary to bear in mind that fossil shells are often pseiidomorphcus, or mere 

 cnsts, 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 mctamorphic. or have undergone a re-arrangement of their particler-, 

 like the rocks in vlacli they occur. 



