MALACOZOA. 17 



colours, variously disposed, which are deposited by the 

 edges of the mantle. 



In a Univalve shell, there are distinguished, the body 

 or larger part ; the spire or tapering portion ; the turns 

 or whorls ; the suture or hue of junction of the turns ; 

 the columella or pillar, the axis of the shell ; the mouth 

 or aperture, with its peristome or margin, which may be 

 complete or not, and may be described as forming an 

 outer lip, and an inner Ujj. The spiral turns may be 

 smooth, or variously marked with striae, laminae, ribs, 

 nodosities, or spines, the markings being longitudinal 

 or transverse. It is to be remembered that, in its natural 

 position, the mouth of the shell is beneath and forward, 

 the spire pointing backwards, and to the right side. 

 Some shells have the mouth on the left side, and are 

 called sinistral, those of the ordinary form being dextral. 



Bivalve shells are those composed of two pieces, kept 

 together by a sort of hinge. When the two valves are 

 equal, the shell is said to be equivalve ; when unequal, 

 inequivalve. They may be round, elliptical, ovate, linear, 

 or of various forms. The more or less prominent part 

 of the valve at the joint is the umbo. When the umbo 

 is nearly in the middle, the shell is said to be isomeral 

 or equilateral ; when not, anisomeral or inequilateral. 

 The hinge may be plain, but generally it presents various 

 prominences, called teeth, with depressions ; the teeth of 

 one valve fitting into the depressions of the other. The 

 valves are further kept together by an elastic fibrous 

 ligament, which tends to throw them open. They are 

 approximated by a pair of strong muscles extended in- 

 ternally from the one valve to the other, and leaving 

 strong impressions on the inner surface. The teeth are 



B 2 



