BIVALVES CARDIUM. 29 



CARDIUM Cockle or Heart-shell. 



DESCRIPTION OF PLATE VII. 



Div. L — Fam. 1. Fig. 3. C. cardissa. 

 DiT. \l.'—Fani.\. Fig. 6. C. retusum. 



Fam. ?. Fig. 8. C. isocardia. 



Fam. 4. Fig. 1. C. medium. — Fig. 4. & 5. C. parvura. 

 DiT. \ll>— Fam. 1. Fig. 7. C. IsBvigatum. 



Fam. 2. Fig. 2. C. serratura. 



Shell bivalve, nearly equilateral, equivalve, generally convex, longi- 

 tudinally ribbed, or striated, with a crenaled margin : hinge with 

 two teeth near the beak, and a larger remote lateral one on either 

 sidCf each locking into the opposite valve. 



THIS genus contains forty-seven species, which vary 

 much in color and structure. The most general shape 

 is convex or orbicular ; some are elongated and 

 compressed ; and others have the form of a flattened 

 heart. 



The general characteristics of this genus are longi- 

 tudinal ribs and grooves, proceeding from the umbo- 

 nes to the margin, and crossed by delicate striae, like 

 the common Cockle, but more distinctly articulated. 

 In some species the surface is smooth and polished; 

 and in others, the ribs are beset with rows of acute 

 spines. The interior margin is almost universally 

 crenated. 



The umbones of the shells of this genus differ consi- 

 derably in position: in some they are placed opposite 

 each other, remote or approximate ; and in others 

 alternating. This variety of appearance has caused 



