32 MANUAL OF THE MOLLUSCA. 
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 conferyoid sponge, hence great care is 
required in determining whether the perforations are an 
essential part of the shell. 
The brachiopoda 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, b 
the middle, and c the inner 
stratum ; they may be seen 
also in Fig. 25. Hach 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 
transverse in the central 
stratum, and lengthwise in the outer and inner (as in cyprea, 
cassis, ampullaria, and bulimus), or longitudinal in the middle 
layer and transverse in the others (e. g. conus, 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.—(Bowerbank.)+ 
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, Brug., from the Miocene of the Touraine. A, 
longitudinal section of a fragment; B, complete horizontal section; a, outer layer; 0, 
middle ; c, inner layer; d, e, f, lines of growth. 
7 It is necessary to bear in mind that fossil shells are often pseudomorphous, ormere 
casts, in spar or chalcedony, of cavities once occupied by shells; such are the fossils 
found at Blackdown, and many of the London clay fessils at Barton. The Paleozoic 
fossils are often metamorphic, or have undergone a re-arrangement of their particles, 
like the rocks in which they occur. 
