BIVALVES^^ARCA. 53 



ARCA Ark. 



DESCRIPTION OF PLATE XIII. 



i)iT. I. Fam. 1. Fig. 1. A. tortuosa. Div. 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. 



DiT. III. Fig. 5. A. nucleus. 



Shell bivalve, equivalve ; hinge with numerous sharp teeth, alter- 

 yiately inserted between each other. 



THE genus Area, 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. noae, and A. 

 barbata, &c.; the suborbicular in the A. glycymeris, 

 &c. (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 rauricated 

 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 



