EXPLANATION OF THE PARTS OF SHELLS. 13 



some species of Venus, &c. From this circumstance it is seldom 

 the highest part of the shell. Plate II. fig. 5. r r. 



Seam. When the valves are closed, the line of separation 

 between them is so called. Plate II. fig. 6. t t. 



Umbo. That part situated immediately under the beak. Plate L- 

 fig. 6. IV ; and Plate 11. fig. 4. w. 



Ears. The processes on each side of the beak, in most species 

 of that division of Ostrea, called pecten; some have one ear very 

 large, and the other small ; and some are scarcely observable on 

 one side. Plate IV. fig. 2. h h. 



Superior Ear. Plate IV. fig. 1 1. /j. 



Inferior Ear. Do. do. i. 



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

 ference of either valve, all round. Plate I. fig. A. p ; and Plate II. 

 fig.]./.. 



Crenulated margin. That fine notched edge of shells, which 

 unite into notches in the opposite valve, as genus Donax. Plate 

 IV. fig. 6. VI m. 



Strise are fine thread-like lines, generally on the exterior surface 

 of shells, and are sometimes both longitudinal and transverse. 

 When the striae of shells appear indistinct, as if worn out, it is 

 termed obsolete strias. In some instances the insides of shells are 

 striated; for example, the Fasciolaria tulipa. The character of 

 the striffi is often of much use in distinguishing species. Plate II. 

 fig. 2./. 



liight Valve, 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 Shell is taken from the ligament, or the beak, to 

 the opposite margin. For example, Mytilus: it is longer than it 

 is broad, and the solens are broader than long. Plate I. fig. 5. 

 li %t. and Plate II. fig. 4. xi ii. 



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

 rior and posterior slopes, being in a contrary direction from its 



