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are beneath the hinge, and terminate in the points or beaks 

 (mucrones) (e), which are incurved, reflexed, earformed, 

 &c. The beaks are frequently, in particular shells, accom- 

 panied by two external impressions ; one of these, the 

 corselet (f), is on the anterior surface, and is separated 

 from the disk, generally, by a ridge, an angle, or a sunken 

 line, and is often distinguishable by its difference of colour 

 as well as by other circumstances : it is sometimes spinous, 

 carinated, lamellated, &c. but it is generally smooth, when 

 it is said to be naked. The other, the lunule (#), is placed 

 at the bottom of the posterior surface : it is variously 

 shaped, oval, oblong, lanceolated, &c. and is sometimes 

 edged, toothed, &c. The shell being placed on the hinge, 

 the anterior side being forward, that is considered as the 

 right valve which answers to the left hand, and that as the 

 left valve which answers to the right hand of the observer. 

 The length of the shell is measured from the hinge to the 

 outer or upper edge, and the width from the end of the 

 anterior to that of the posterior margin. Those shells, 

 whose length exceeds their width, are considered as longi- 

 tudinal shells, and those whose width exceeds their length, 

 transverse shells. The forms and markings of bivalves are 

 too numerous to be noticed in this place. Shells are ne- 

 cessarily distinguished into free, such as are capable of 

 being moved, and fixed., being such as are adherent to other 

 solid bodies. On the internal surface of the valves are 

 impressions of a regular form, varying in that respect as 

 well as in their number, in different shells : these are the 

 places where the animal was attached to the shell by its 

 muscular or tendinous attachment, and may be called the 

 muscular impressions. These vary in their number, ac- 

 cording to the number of muscles employed for this purpose, 

 being one, two, or even more, on each valve. The hinge (h) 

 is placed in the most solid part of the shell, generally in its 

 base, beneath its beaks. It is usually formed of teeth, 



