348 MOLLUSCA SUB-KINGDOM VI 
structure. The external layer is usually thin, flexible, and dark-coloured, 
chiefly composed of a horny substance termed conchiolin. This layer is 
known as the epidermis, or more properly the periostracum ,; it is not easily 
corroded, and hence serves as a protection to the underlying calcareous 
layers. The outer calcareous layer is composed of prisms of calcite arranged 
more or less perpendicular to the external surface; the inner layer is made 
up of thin, more or less parallel lamellae of porcellanous or pearly texture, 
disposed at right angles to the general direction of the prismatic layer, 
and exhibiting the minera- 
logical characters of ara- 
gonite (Fig. 590). Besides 
the lamellar or prismatic 
structure, many forms 
show under the micro- 
scope minute, sometimes 
branched tubulation. 
The variations in shell 



Fic. 590. 
Vertical section of the shell of Unio. e, b, a, a’, the outer prismatic substance are somewhat 
layer, showing successive increments of shell growth; ¢, ¢’, the inner characteristic of different 
lamellar strata. Highly magnified (after Carpenter). ‘ 
groups. The prisms vary 
greatly in size, the larger occurring in IJnoceramus and Pinna, the smaller 
in the Anatinidae and Myacidae. The prismatic layer is wholly absent in 
the Chamidae and many other Teleodesmacea ; in the Pectinidae and Limidae 
the prismatic layer is feebly developed and often recognisable only in young 
shells. In the Rudistae the prisms run nearly parallel with the outer surface. 
As aragonite is more soluble than calcite, it frequently happens in fossil 
shells that the layers composed of the former mineral have entirely dis- 
appeared, leaving only the calcitic layers. Pearls are merely loose portions 
of the inner layer secreted by the mantle surface, usually around foreign 
bodies which have reached the interior of the shell and set up irritation there. 
In the majority of Pelecypods the valves form a nearly complete defence ; 
in borers, burrowers, and a few degenerate types, the valves cover less and 
less surface in proportion to that which is bare; in a few the mantle is re- 
flected so as to envelop more or less of the outer surface of the valves; and 
finally, in Chlamydoconcha, the valves are permanently internal, separately 
encysted, with the ligament isolated and encysted between them. No 
example is known of a Pelecypod absolutely destitute of valves in the adult 
state. 
The valves of the shell are in general substantially equal; but sometimes 
they are unequal, especially in sessile or sedentary forms; and rarely they 
are spirally twisted, as in Stavelia and Spirodomus. The hinge or articulus 
comprises the whole articulating apparatus,—hinge plate, teeth, ligament, ete. ; 
the primitive hinge, which is coextensive with the ligament, is distinguished 
by Hyatt as the cardo. The cardinal axis, or right line forming the axis of 
revolution of the hinge, is parallel with the antero-posterior axis of the 
animal (as determined by a line drawn through the mouth and posterior 
adductor) in the ordinary Teleodesmacea ; but in the winged Prionodesmacea, 
such as Ostrea, Perna, etc., the two axes are at a considerable angle with each- 
other. 
The dental armature is usually situated on the dorsal margin, which for 
