MOLLUSCA 191 
(i.) the periostracum, or outermost layer—this is thin and 
horny, and not calcified, and is formed by the thickened free 
edge of the mantle; (i1.) the prismatic or middle layer, con- 
sisting of closely-packed calcareous polygonal prisms—this is 
also deposited by the edge of the mantle; (iii.) the nacreous 
or mother-of-pearl layer, which lines the inside of the shell— 
it is composed of laminae of calcareous matter, and is de- 
posited by the whole of the surface of the mantle and body 
in contact with the shell. It is this last layer which, when 
deposited in concentric layers round foreign particles, such 
as grains of sand, etc., produces pearls. 
The shells of some Lamellibranchs are not equivalve, e.g. 
the oyster, Ostrea, which is attached to rocks by means of 
its larger valve. In Pholas there are additional calcareous 
plates inserted dorsally between the two valves; and in 
Teredo, the mollusc which does so much damage by boring 
into wood, the valves fail to completely cover the body, 
which secretes a calcareous lining to the tube in which it 
lives. 
The valves of the shell are kept in apposition by adductor 
Fria. 116.—Section through Anodonta, to show mechanism of 
opening and of closing the valves. After Lankester— 
Zoological Articles reprinted from the Encyclopedia 
Britannica. 
1. Right valve of shell. 
2. Left valve of shell. 
. Hinge. 
em ch 
. Elastic ligament. 
Or 
. Adductor muscles. 
muscles. These may be two in number, an anterior and a 
posterior, or the posterior may alone persist (Monomyaria). 
The edge of the mantle is thickened, and in some genera 
it bears tentacles and eyes. Posteriorly it is notched in such 
a way as to form two apertures, which remain open when the 
