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THE LAMELLIBRANCHIA 213 
The shell of Lamellibranchs is formed of two valves, each 
corresponding to a lobe of the mantle. The internal layer of the 
shell is secreted by the whole external surface of the mantle, but 
the external layer is secreted only by the thickened mantle edges. 
The internal layer is often nacreous, and may exhibit pathological 
products called “pearls,” which are formed by the secretion of 
nacreous substance by the mantle round foreign bodies. These 
are generally of parasitic origin: the scolex of a Cestode forms the 
nucleus of a pearl in the genus Meleagrina of Ceylon and the Gambier 
islands ; the larvae of 'rematodes form similar nuclei in various 
European Lamellibranchs. 
Though they are primitively symmetrical and commonly remain 
so, the valves become very asymmetrical in some species of Arca, 
in the Anomiidae, Pecten, Ostraea, Corbula, Chama, Pandora, Myochama, 
the Rudistae (Fig. 244), ete. In certain somewhat modified forms 
in which the foot, though more or less large, is feebly retractile, 
the valves do not meet and fit perfectly together along the ventral 
edge and are “ gaping,” as may be seen in the Pholadidae, Gastro- 
chaenidae, etc. But with the exception of Chlamydoconcha and 
Scioberetia, in which the shell is internal, the valves fit together 
perfectly along the dorsal border, and are articulated with one 
another by a system of teeth and sockets which collectively form 
the hinge (Fig. 189), and only tend to be atrophied in forms whose 
valves have little mobility, especially in boring species. The valves 
are additionally united (except in the Pholadidae and Teredinidae, 
hence named Adesmacea, and a few other forms) by a ligament of 
a chitinous nature. This ligament is primitively continuous with 
the shell, and is, in fact, the uncalcified portion of the pallial cuticle, 
that is to say, of the originally single shell. The ligament finally 
becomes external (Fig. 189) or internal; in the latter case it is a 
“resilium.” Its action is antagonistic to the adductor muscles, and 
consequently it causes the valves of the shell to gape. 
In the youngest stages of the Protobranchia, Filibranchia, and 
various Eulamellibranchia, a series of little transverse denticulations, 
constituting a primitive hinge or provinculum (Bernard), is developed 
on each side of the ligament, or at any rate behind it in forms 
devoid of an anterior adductor muscle. The permanent hinge teeth 
are only formed at a later period, by the growth of distinct laminae 
on the surface of the hinge. Thus, in the typical Eulamellibranchia, 
the first lamellae originate at the extremities of the hinge surface, 
below the provinculum, and grow towards the centre of the hinge 
area; the internal ends of the anterior lamellae become hook- 
shaped, and their hooks become separated from their external ends ; 
the latter form the anterior lateral teeth, while the hooks become 
the cardinal teeth, and the posterior lamellae give rise to the 
posterior lateral teeth. 
