20 INTRODUCTORY. 



Margin. The extreme edge of the whole shell, or the circum- 

 ierence of either valve, all round. Plate I. fig. 4. p ; and Plate 

 2. fig. 1. p. 



Crenulated margin. That fine notched edge of most of the 

 cockles, which unite into notches in the opposite valve. It is 

 by no means uncommon in other shells, particularly those of the 

 genus Donax. Plate IV. fig. 6. m m. 



Strioe are fine thread-like lines, generally on the exterior sur- 

 face of shells, and are sometimes both longitudinal and trans- 

 verse. When the striae of shells appear indistinct, as if worn 

 out, it is termed obsolete strice. In some instances the insides 

 of shells are striated ; for example, the Murex tulipa. (Fasciolaria 

 tulipa, Lamarck.) The character of the striae is often of much 

 use in distinguishing species. Plate II. fig. 2.f. 



Right Value is that valve which, when viewed with the inside 

 uppermost has the anterior slope pointing to the right hand. 

 Plate I. fig. 8 ; and plate 4. fig. 5. and 6. b. 



Left Valve. The opposite of the above; the anterior slope 

 points to the left hand, when viewed from the inside. Plate 

 IV. fig. 3, 6, 7, 9. &c. 



Length of the ShsUis taken from the ligament, or the beak, 

 to the opposite margin. For example, the common muscle : it 

 is longer than it is broad, and the solens are broader than long. 

 Plate I. fig. 5. u u. and Plate 2. fig. 4. u u. 



Breadth is measured from the most extreme edge of the an- 

 terior and posterior slopes, being in a contrary direction from 

 its letigth. Many shells are broader than long, such as most of 

 t(^e Myse, Solenae, Tellinae, &c. ; and the Mytilus, Ostrea, Pin 

 iia, &c. are in general longer than broad. Plate I. fig. 5. v v. 

 Plate II. fig. 4. w u, 



Byssus or heard, is an appendage composed of filaments of a 

 silky texture, by which some of the Bivalves fasten themselves, 

 to their beds, such as the Muscles, PinnaB, &c. Plate II. fig. 6. s. 



