FAMILY ARCADZ — ARCA. 177 
ARCA TRANSVERSA. 
PLATE XII. FIG. 212. 
Arca transversa. Say, Journ. Acad. Nat. Sciences, Vol. 2, p. 269. 
A, id. Govtp, Invertebrata of Mass p. 96. 
Description. Shell smaller than the preceding, thick, transversely oblong, subrhomboidal. 
Surface with from thirty-two to thirty-five strong radiating ribs, obsolete on the beaks, and 
crossed towards the lower margin by two or more concentric furrows of growth: these ribs 
are nearly their own diameters apart, and become larger near the margin. Beaks prominent, 
meurved, and separated by a long and narrow interval: they are placed at the end of the 
anterior third of the length of the hinge-margin. Valves slightly unequal, so that the margin 
of one passes slightly beyond the other ; this is most conspicuous on the posterior portions of 
the lower margin: a slight curve at each extremity of the hinge-margin. One or more an- 
gulated lines on the hinge space, drawn from the beaks to the hinge edge: valves accurately 
closing all round. 
Color. Dingy white, sometimes tinged with reddish, and particularly adherent about the 
lower margin. Epidermis chesnut-brown, foliaceous. 
Length, 0°5-0°8. Transverse diameter, 1°0-1.4. 
This is also a very common species on our coast. It ranges north nearly to Cape Cod, 
and occurs on the coast of New-Jersey. 
(EXTRA-LIMITAL.) 
A. ponderosa, (Say, Ac. Sc. Vol. 2, p. 267.) Shell very thick and ponderous, somewhat oblique, with 
25 -- 28 ribs, each marked with an impressed line. Beaks distant, and opposite the middle of the 
hinge: lower margin nearly straight, or contracted in the middle. Length, 2; transverse diameter, 
2°5. Southern Coast. 
A. incongrua. (Ip. Ib. Vol. 2, p. 268.) Shell somewhat rhomboidal, with 26 - 28 ribs, nearer than 
their own diameters, and crossed by elevated obtuse equal and equidistant lines, which are altogether 
wanting on ten rays of the disk of the left valve. Beaks distant, opposite the middle of the hinge, 
with a lanceolate space between: anterior margin cordate, flattened. Allied to A. rhombea, Born. 
Length, 2°0; transverse diameter, 2°1. Southern Coast. 
Fauna — Part 6. 23 
