EXPLANATION OF THE TARTS OF SHELLS. 23 



Umbo That part situated immediately under the beak. 



Plate I. fig. 9, w; and Plate II. fig. 4, iv. It is properly 

 the most prominent part of the shell, near the hinge, and of 

 which the beak is the tip. 



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

 species of that division of Ostrea, called scallops; some 

 have one ear very large, and the other small ; and some are 

 scarcely observable on one side. Plate II. fig. 2, // h. 



Superior Ear. — Plate IV. fig. 11, h. 



Inferior Ear Plate IV. fig. 11, i. 



Margin. — The extreme edge of the whole shell, or the 

 circumference of either valve, all round. Plate I. fig. 7, p ; 

 and Plate II. fig. 1, p. 



Creniilated Margin When the margins are notched, or 



marked with alternate prominences and depressions, which 

 often interlock in the opposite valves, as in Cockles. It is 

 by no means uncommon in other shells, particularly those 

 of the genus Donax. Plate IV. fig. 6, m m. 

 . Strice 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, they are termed obsolete strice. In some in- 

 stances the insides of shells are striated ; for example, the 

 Murex Tulipa. (Fasciolaria tulipa, Lamarck.) The char- 

 acter of the striae is often of much use in distinguishing 

 species. Plate II. fig. 2, /. 



Right Value is that valve which, when viewed with the 

 inside uppermost, has the anterior slope pointing to the right 

 hand. Plate I. fig. 11; and Plate IV. 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 



* But the only sure way of knoAvinj^ which is right or left is to 

 examine the animal, and place the shell in its natural position, that is, 

 with the hinge above, an_d the ventral margin, I'lato I. fig. 7, P, beneath ; 

 in which ease the ligament is behind the umboncs, so that what is 

 usually termed right is actually left. 



