EXPLANATION OF THE PARTS OF SHELLS. 17 



Bivalve shells consist of two parts or valves, connected by a 

 cartilage, and a hinge which is generally composed of teeth, those 

 of the one valve locking into a cavity in the other. 



The valves of some bivalve shells are formed exactly alike, 

 and others are very different ; the one being smooth, the other 

 rugose ; one flat, and another convex ; and often one is shorter 

 than the other. 



The shells of the Mya, Solen, Tellina, Venus, and others, have 

 in general both valves alike, while those of the Spondylus, Ostrea, 

 Anomia, and Pinna, have in general dissimilar valves. The first 

 of these kinds are called equivalve, and the latter inequivalve. 



Equilateral shells, are those whose sides are alike, as in 

 the shells of the genus Pecten, commonly termed Scallop shells 

 in England, or those shells worn by pilgrims in former times ; 



" And fix'd the Scallop on his hat before." 



Parxell. 



Plate II. fig. 2. and plate VII. fig. 14. This is also exempli- 

 fied in the Pectunculus. 



Inequilateral valves, are shells whose sides are unequal ; and of 

 different shapes, as in the Mactra, Donax, &c. 



Summit, is the most elevated point of that part of the shell in 

 which the hinge is placed. Plate I. fig. 4. and 8. a a. 



In naming this the summit, we do not follow the axiom of 

 Linnaeus, but because we consider it more properly the summit 

 of the shell, than the opposite extremity. 



Base, is the reverse of the above, or that part of the shell 

 immediately opposite the summit. Plate I. fig. G. and 7. dd. 



Sides, the right and left parts of the valves. Plate I. fig. 6. c. 



Anterior slope, is that part of the shell in which the ligament 

 is situated. In viewing the anterior slope in front, the beaks of 

 the shell retire from view. Plate I. fig. 4. i. 



Posterior slope, that part of the shell opposite the anterior 

 slope ; in viewing it in front, the beaks point to the observer. 

 Plate I. fig. 4.. k. 



Disk, the convex centre of a valve, or most prominent part or 



B .J 



