BIVALVES — ARCA. 53 
ARCA. ARK. 
DESCRIPTION OF PLATE XII. 
Div. I. Fam. 1. Fig. 1. A.tortuosa. Drv. II. Fam. 2. Fig. 6. A. pectunculus. 
Fam. 2. Fig. 4. A. lactea. Fam. 2. Fig. 7. A. glycymeris. 
Fam. 5. Fig. 2. A. barbata. Fam. 4. Fig. 3. A. granosa. 
Dw. III. Fig. 5. A. nucleus. 
Shell bivalve, equivalve; hinge with numerous sharp teeth, alter- 
nately inserted between each other. 
THE genus Arca, of which there are forty-one species, 
is easily distinguished from other genera of Bivalves 
by the peculiarity of the hinge; which, without any ex- 
ception, is composed of numerous sharp teeth, alter- 
nately inserted between each other. The line of direc- 
tion of the hinge is in some species perfectly straight, 
and in others it is arched or curved. These characters 
form the distinctions of the first and second divisions. 
The forms of the Arks vary exceedingly: the elong- 
ated is exemplified in the A. tortuosa, A. noz, and A. 
barbata, &c.; the suborbicular in the A. glycymeris, 
&e. (which are nearly smooth on the outside, and 
have the inner margin generally crenated); and the 
subcordate, in the A. senilis, &c. which are also some- 
what gibbous, and have usually smooth or muricated 
grooves. The shells of this genus are covered with 
a brownish or greenish-black epidermis. 
Many of the Arks gape at the superior margin; others, 
on the contrary, are perfectly close. Some have the 
margin entire, others are crenulated, and several have 
