464 



TERMINOLOGY OF 



a 



Beaks. — These constitute the apex of the shell : they are 

 prominent points looking to each other, or, more generally, 



inclined to the anterior 

 "'£• 85 - slope. They have a ten- 



dency to the spiral figure, 

 and terminate the supe- 

 rior, or dorsal margin of 

 the shell. When they 

 are situated decidedly in 

 front, they become ter- 

 minal, as in the mussels ; 

 when only approximat- 

 ing to the anterior slope, 

 they are said to be late- 

 ral ; and when near the 

 middle, they are central. They are sometimes obsolete, or 

 scarcely distinguishable, as in Solen ; they are very large, 

 and heartshaped, in some cemented shells, as Chama, Diceras, 

 and Gryphus, and they are spiral in Isocardia. In almost 

 all genera the beaks have the same inclination ; but in 

 some Cham as they look in opposite directions, and are then 

 divergent. 



The Lunule (Fig. 85, &) is not to be found in every bivalve. 

 It is seldom observable in the Monomyaires, but very often 

 in the Dimyaries, and is most developed in the Veneridae. 

 It is a space in the anterior slope, placed immediately under- 

 neath the beaks, and usually circumscribed by a line or de- 

 pression. Its figure is variable, — either heartshaped in glo- 

 bular and ventricose shells, or lanceolated in flattened shells. 

 The lunule is rarely protuberant, except in its centre, but it 

 usually constitutes a more or less depressed or hollow area, 

 circumscribed, in some cases, by a raised margin. The cen- 

 tral line is either close or gaping. 



The Corselet (Fig. 85, c) occupies a portion of the superior 

 and posterior slope of the shell. It is only found in the 

 Dimyaires, and is wanting in many of them, nor is it so dis- 

 tinctly defined as the lunule. It is at its upper part that 

 we find the labia and nymphai in such bivalves as have an 

 external ligament. To gain a distinct idea of the corselet, 

 you must examine it in the fossil Trigoniae, and in the Venus 

 dione of Linnams, — a common and singular shell, indispens- 

 able to your collection. The corselet is in general circum- 

 scribed by an angle, a keel, or a series of tubercles, scales, or 

 spines. It is sometimes narrow, and deep, or lanceolate, 

 heartshaped, or oval; and it may be ornamented like the 

 general surface. It is said to be escutcheoned when it is di- 



