Lamarck’s Genera of Shells. 319 
elongated, whence their name. They often gape at the superior 
margin, in consequence of the tendinous fibres which the animal 
puts out at that part to fix them to the rocks. 
The space between the beaks forms a romboidal, flat, or some- 
what hollow facet, which receives the ligament. The facet is 
marked with furrows, which form lozenge-shaped figures when 
the valves are closed. Two muscular impressions are visible at 
the sides, in the interior of the shell. 
The Arce live near the coasts, some buried in the sand, 
others on the surface. Several have a scaly or velvety epi- 
dermis. 
This species is subdivided into (1) shells whose superior 
margin is not crenate within,—23 species; and (2) those with 
the superior margin internally crenate,—14 species. 
Type. Arca tortuosa*. (Idem. Linn.) 
Shell distorted, parallelopipedal, striated ; valves obliquely 
carinated; nates small, curved. Indian Ocean. In all 37 recent 
species, and 9 fossil. Pl.vi. Fig. 65. 
3. Pectunculust. 
Shell orbicular, almost lenticular, equivalve, subequilateral, 
close. Hinge arched, furnished with numerous serial, oblique, 
entering teeth, the middle ones obsolete, nearly obliterated. 
Ligament external. 
The pectunculi are distinguished from the arce by the orbi- 
cular form of their shells, and especially by the hinge being arch- 
ed, instead of straight ; the teeth also are less numerous, farther 
apart, and larger, and they never gape. The foot of the ani- 
mal appears to be securiform, and lobed. It has no projecting 
trachez. The beaks of the shell are not very distant from each 
other, yet are always separated by an external, narrow, angu- 
larly-furrowed, rather hollow facet, to which the ligament is 
attached, and which distinguishes the pectunculi from the nu- 
cule, whose ligament is partly interior, and which have no facet 
between the beaks. 
* Crooked. 
+ An original term, the name of a sort of shell-fish. 
