TRIBE ARCACEA. 151 
obliquely grooved, the valves carinated; anterior side the shorter, 
smooth and depressed. An. Var.? Son. G. f. 3.—Reeve. 
t. 78. f. 3. Ribs closer and as well as the umbonal ridge, more 
obtuse. 1.—China and the Indian Ocean. 
I. Semisutcata. Lam. 4.  Heart-shaped, thin, semi-trans- 
parent, white, transversely striated; anterior side with ten longi- 
tudinal grooves.® 

TRIBE ARCACEA. 
Primary teeth small, numerous, and disposed in a line in each 
valve, which is either straight arched or broken, the teeth of 
one valve fitting into the intermediate spaces of the other valve. 
CUCULLEA. 
Equivalve, inequilateral, trapeziform, ventricose ; beaks distant, 
separated by the area of the ligament; anterior muscular 
impression elevated, its margin angulated, or mith an eared 
produced border ; teeth of the hinge straight, very small, and 
set transversely, with from two to five small parallel ribs ; 
ligament external. 
C. Auricutirera. Lam. 1.—Sow. G.—Reeve. t. 81. f. 1, 
2, 3.—Arca CucuLLtEA. Gmel. 3311.—W. t. 9. f. 16.—A. 
ConcaMERATA. WD. p. 232.—E. t. 304.—Ch. f. 526, 7. Ob- 
liquely heart-shaped, rhombic, cinnamon colour, ventricose, decus- 
sated by striz of which the longitudinal ones are the stronger ; 
within whitish, stained anteriorly with reddish or pale violet brown, 
with a laminar appendage proceeding from under the beak; hinge 
with two large teeth at each extremity. 2}..34.—Nicobar Isles. 
ARCA. 
Transverse, subequivalve, inequilateral ; beaks remote, separated 
by the area of the ligament ; hinge linear, straight, without ribs 
at the extremities ; teeth of the hinge numerous, arranged in a 
row, serrated, close set, alternately inserted into opposite valves ; 
ligament entirely external. 

* For this species Deshayes proposes the establishment of the 
genus CarpiLIA. Ovyal-oblong, longitudinal, heart-shaped, ven- 
tricose, the beaks large and prominent; hinge with a small car- 
dinal recurved tooth, a pit at the side and a spoon-shaped pro- 
jection for receiving an internal ligament: anterior muscular scar 
rounded and superficial, the posterior placed on a thin horizontal 
plate which projects from beneath the hinge. 
