318 Lamarck’s Genera of Shells. 
1. Cucullea*. 
Shell equivalve, inequilateral, trapezoidal, ventricose ; beaks 
distant, separated by the facet of the ligament. Anterior mus- 
cular impression elevated; margin angular, or auriculated. 
Hinge linear, straight, with very small transverse teeth ; from 
two to five ribs at the extremities, parallel tothe hinge. Liga- 
meut wholly external. 
The cucullee are large, very tumid, trapezoidal shells, with 
the anterior side obliquely truncated, forming a broad, cordate, 
flattened corselet, slightly elevated towards the middle. The 
hinge is that of thevarcee, but as the shell grows it is displaced, 
and leaving the remains of its former margins at the extremities, 
gives rise to two parallel ribs, by which it is terminated, which 
is not seen in the arce. These singular ribs have a very dif- 
ferent direction from that of the serial teeth of the hinge, and 
cannot be taken for teeth. When old, these shells are very 
thick, and the lateral ribs of their hinge become progressively 
more numerous. The facet of the ligament also increases in 
proportion, and becomes more furrowed. 
Type. Cucullea auriculiferat. (Arcacucullus. Gmel.) 
Shell obliquely cordate, ventricose, decussately striated, 
yellow; hinge on both sides subbicostate. Indian Ocean. 2 Spe- 
cies. PI. vi. Fig. 64. 
2. Arcaft. 
Shell transverse, subequivalve, inequilateral, beaks distant, 
separated by the facet of the ligament. Hinge linear, straight, 
no ribs at its extremities, and furnished with numerous serial, 
entering teeth. Ligament wholly external. 
The Arce are sea shells, and readily known by the peculiar 
form of their hinge. They are generally very inequilateral, 
almost rhomboidal, and remarkable for the distance between 
the beaks of most of them. When placed on the superior mar- 
gin they resemble a boat, especially those which are transversely 
* From Cucuillus, a hood. + Having cars ; auriculated. 
+ A Chest or Ark. 
