564 MOLLUSCA 



The shell, like the mantle, consists of two parts or valves, which 

 are joined together dorsally and open ventrally to permit the edges of 

 the mantle, the siphons, and the foot to be protruded. The shell is 

 invariably present, but may be rudimentary; the two valves are not 

 always symmetrical, and vary much in thickness in the different species. 

 The outer layer or periostracum is thin and non-calcareous and is often 

 wanting. The inner or mother-of-pearl layer shows a play of colors in 

 many species, due to the refraction of light by the delicate horizontal 

 plates of which it is composed, which often gives the shell commercial 

 value. Pearls are cysts of mother of pearl which have formed around 

 foreign objects, usually larval trematode worms, which have lodged 

 between the mantle and the shell. The middle or columnar layer is usu- 

 ally thick. The mother-of-pearl layer is de- 

 posited by the general surface of the mantle. 

 The other two layers are formed by its margin, 

 and hence show concentric lines of growth. 

 These lines are grouped around the umbo or 

 beak at the dorsal margin of the shell, which 

 is the oldest part of it, and often prominent, 

 and often projects forwards. In front of it 

 is, in certain bivalves, a heart-shaped de- 



P reSSed area Called the ? Unule < Fi &' 889 > 4 )- 

 The tw ValveS f the she11 are held 

 ^ a m re r leSS elastic band Called the 



ment (2) which is usua11 ^ external and con - 



sists f tw distinct P arts > the outer Portion 



10, pallial sinus. or cartilage, the latter being very elastic and 



composed of parallel fibers. 



Beneath the umbo is the hinge, which is usually composed of inter- 

 locking teeth in the two valves. These teeth are distinguished as cardi- 

 nals and laterals, the former (7) being immediately beneath the umbo; 

 the latter often consist of long ridges and are called the posterior lat- 

 erals (8) when they are behind, and the anterior laterals (6) when in 

 front of the umbo. The inner surface of each valve shows, often promi- 

 nently, a number of impressions caused by the attachment of muscles in 

 the shell. These are the adductor muscle impressions (1 and 5), of 

 which there are either one or two, the much smaller impressions of the 

 siphonal and pedal retractor muscles, and the pallial line (9). The lat- 

 ter is a broad line which connects the anterior and posterior adductor 

 impressions and represents the points where the numerous retractor 

 muscles of the margin of the mantle are inserted; it is often indistinct. 



