BIVALVES__CARDIUM. 29 
CARDIUM_Cockte or HEART-SHELL. 
DESCRIPTION OF PLATE VII. 
Div. I. —Fam. 1. Fig. 3. C. cardissa. 
Dw. II. —Fam.1. Fig. 6. C. retusum. 
Fam. 2. Fig. 8. C. isocardia. 
Fam. 4. Fig. 1. C. medium.---Fig. 4. & 5. C. parvum. 
Div. I1L—am. 1. Fig. 7. C. levigatum. 
Fam. 2. Fig. 2. C. serratum. 
Shell bivalve, nearly equilateral, equivalve, generally convex, longi- 
tudinally ribbed, or striated, with a crenated margin: hinge with 
two teeth near the beak, and a larger remote lateral one on either 
stde, 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 strie, 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 
