MOLLUSC A 



191 



(i.) the periostracum, or outermost layer — this is thin and 

 homy, and not calcified, and is formed by the thickened free 

 edge of the mantle ; (ii.) 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 LameUibranchs 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 dorsaUy 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 



Fig. 116. — Section through ^jiof^onto, to show mechanism of 



opening and of closing the valves. After Lankester — 



Zoological Articles reprinted from the Uncydopcedia 

 Britannica. 



1. Right valve of shell. 



2. Left valve of shell. 



3. Hinge. 



4. Elastic ligament. 



5. 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 



