20 INTRODUCTION. 



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."— Parnell. 



Plate II. fig. 2, and Plate VII. fig. 14. This is also exem- 

 plified in the Pectunculus. Plate XVI. fig. 8. 



Inequilateral Valves are shells whose sides are unequal, 

 and of different shapes, as in the Mactra, Donax, &c. Plate 

 XVII. fig. 5. 



Summit, or Umbo, is the most elevated point of that part 

 of the shell in which the hinge is placed. Plate I. fig. 7. 

 and 11, a a. 



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

 of Linntcus, 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. tig. 9. and \Q,dd. 



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

 fig. 9, c. 



Anterior slope is that part of the shell in which the liga- 

 ment is situated. In viewing the anterior slope in front, 

 the beaks of the shell retire from view. Plate I. fig. 7, i* 



Posterior slope, that part of the shell opposite the ante- 

 rior slope ; in viewing it in front, the beaks point to the 

 observer. Plate I. fig. 7, k." 



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

 part of the valve, suppose it with its inside lying undermost. 

 Plate I. fig. 7, o. 



Inside, the concave part of a valve. Plate IV. fig. 11, n. 



Cicatrix, or Tongue, is the impression left on the inside 

 of the valves, by the adhering muscles of the animal. It 

 differs in most shells, according to the shape of the muscle, 

 as semi-ovate, round, lunate, elongated, &c. As a specific 

 distinction, it is often of great use ; being, with a very few 



* But what is thus commonly named anterior, is in fact posterior, 

 as is knowTi by examining the animal, of which the mouth is placed 

 toward h, and the siphons toward i. 



