298 MOLLUSCOIDEA SUB-KINGDOM V 
of the shell cavity with sediment. Not infrequently the shell and the 
brachidia are secondarily replaced by quartz ; and if the interior filling matter 
be dissolved away by dilute hydrochloric acid, exquisite preparations may be 
obtained, often revealing the minutest details. Sometimes hollow shells are 
found, in which the brachidia are well preserved; but these structures 
generally are more or less encrusted. It is often necessary to remove the 
ventral valve, when the infiltrated material can be cut away by the use of 
proper tools. Success in manipulations of this kind requires not only con- 
siderable dexterity, but the conditions of preservation must have been very 
favourable. The brachidia must be perfectly preserved, and the surround- 
ing matrix must admit of being removed without injury to the specimen. 
When other expedients fail, recourse can still be had to polishing, the shell 
being gradually ground down by abrasion with emery powder on a glass 
plate. The beaks are first ground away, until the first traces of the crura 
appear; the surface is then cleaned and kept moistened while a careful 
drawing is made. Grinding is resumed for a short interval, when the 
surface is again cleansed and drawn. This process is repeated until the 
sections include the entire brachial support. From the series of transverse 
sections thus obtained, the brachidium can be ideally reconstructed. 
The spondylium is an internal ventral plate traversing the posterior 
portion of the valve (Fig. 538, C). On the superior surface of the plate are 
inserted the adductors, diductors, and the ventral pedicle muscles. Beneath 
the spondylium, which may be supported by a median septum, are situated 
the reproductive organs. The plate is homologous with the solid or ex- 
cavated platform of the Atremata (Trimerellidae and Linguwlasmatidae). 
The Animal: JMJantle.—Lining the entire inner surface of the shell is a 
thin, transparent membrane, which appears in the embryonic condition as 
two distinet lobes of the thoracic segment in the 
Cephalula stage. This is the mantle or pallium, 
which is primarily concerned in the secretion of the 
shell. In Crania it consists of three layers: a middle 
cartilaginous, an inner ciliated one, and an outer 
layer of cells. The layer lying against the surface 
of the shell is often studded with minute caeca or 
blind tubes, which enter the perforations of the test. 
The mantle (or. certain of its layers) is folded upon 
rig Ab. itself at various points, enclosing cavities or pallial 
Cumarophoria Humbletonensis, sinuses, which contain the circulating fluids, and 
Bneland, ‘Cast showing impres, £kequently portions of the genital organs. Distinct 
sions of pallial sinuses (after impressions of these sinuses are often observable in 
the valves of both recent and fossil specimens (Fig. 
495). In all the greater sinuses of the mantle, in the perivisceral cavity, 
and in the cavernous brachia and cirri, occur calcareous spicules of various 
shapes. These are especially abundant in the Thecidiidae, and form an 
irregular mass or network. They appear to be absent in Jagellania, 
Terebratella, and Lingula. The outer margins of the mantle are thickened 
and set with numerous, chitinous, simple, or barbed sefae, sometimes of great 
length. 
The shell cavity is divided by a vertical membranous wall, which is an 
extension of the mantle, into two regions: a posterior, or visceral cavity, 




\ 
<A 
