GENERA OF SHELLS. 33 



of the ligament. Hinge linear, straight, with- 

 out ribs at the extremities ; teeth numerous, 

 crowded, alternately inserted into each other ; 

 hgament exterior. 



The area is readily known by the pecuhar form of 

 its hinge. When placed on the superior margin, they 

 resemble a boat, whence their name. They often gape 

 at the superior margin, in consequence of the tendinous 

 fibres which the animal puts out to fix himself to the 

 rocks. The space between the beaks forms a rhom- 

 boidal flat, or sometimes a hollow facet, marked with 

 furrows, which receives the ligament. 



(a.) Superior margin not crenulated 2cithin. 



A. tortuosa, semitorta, Noae, tetragona, umbonata, sinuata, 

 avellana, cardissa, ventricosa, retusa, sulcata, ovata, barbata, 

 fusca, Magellanica, Domingensis, lactea, trapezina, pista- 

 chia, pisolana, cancellaria, callifera, irudina. 



(b.) Superior margin crenulated tcithin. 



A. Kelbingii, scapha, antiquala, rhombea, granosa, auric- 

 ulata, inaequivalvis, Indica, senilis, Brasiliana, corbicula, 

 squamosa, Cayenensis, bisulcata — staminea, lienosa, pon- 

 derosa, pexata, incongrua, transversa. 



Pectunculus. Shell orbicular, nearly lentic- 

 ular, equivalve, subequilateral, closed. Hinge 

 arcuated with numerous oblique, uniformly 

 arranged, alternately inserted teeth ; those in 

 the middle obsolete, almost wanting ; ligament 

 exterior. 



The pectunculi are distinguished from the area by 

 the orbicular form of the shells, and especially by the 

 arched hinge ; the teeth are less numerous, farther 

 apart and larger, and they never gape. The beaks are 



