ARCID A. 253 
Shell equivalve or nearly so, thick, subquadrate, ventricose, 
strongly ribbed or cancellated; margins smooth or dentated, 
close or sinuated ventrally ; hinge straight, teeth very numerous, 
transverse ; umbones anterior /separated by a a flat lozenge-shaped 
ligamental area, with numerous cartilage-grooves; pallial line 
simple; posterior adductor impression double ; pedal scars two 
the posterior elongated. 
Animal with a long pointed foot, heeled, and deeply grooved ; 
mantle furnished with ocelli; palpi 0; gills long, narrow, less 
striated externally, continuous with the lips; hearts two, each 
with an auricle. 
The name Byssoarca was chosen unfortunately by Swainson, 
for the typical species of the genus, in which the byssal orifice 
is sometimes very large. The by ssus is a horny cone, composed 
of numerous thin plates, occasionally becoming solid and calca- 
reous ; it can be cast off and reformed with great rapidity. The 
Areas with close valves often have the left valve a little larger 
than the right, and more ornate. 
The Byssoarks secrete themselves under stones at low-water, 
in crevices of rocks, and the empty burrows of boring mollusks ; 
they are often much worn and distorted. 
A large number of genera and subgenera have been formed 
out of the Linnean Area, but as in Helix, Murex and other 
well-known and well-characterized groups, these groups possess 
but slight value; the most important of them may be considered 
subgenera, perhaps; others as sections. 
ARCA, Linn. (typical). Shell oblong, subquadrangular, gaping 
anteriorly or inferiorly ; hinge linear, straight, formed ofa large 
number of small pectinated. teeth ; ligament. external, inserted 
upon a lozenge-shaped area between the beaks ; beaks high, 
rather wide apart ; muscular impressions very distinct ; pallial 
impression entire. A. Nox, Linn. The Arcas often " anchor 
themselves by means of a strong byssus to rocks or stones. 
BARBATIA, Gray, 1840. Shell oblong, oval or subquadrangular ; 
surface covered by a rude, caducous epidermis; hinge-line 
straight or somewhat curved : teeth numerous, the central ones 
smallest, the lateral ones becoming gradually ‘larger and more 
oblique towards the extremities; ligament external, inserted 
upon a narrow surface between the approaching beaks. A. velata, 
Sowb. (cxxvi, 40). 
POLYNEMA, Conrad, 1875. Shell transversely trapezoid-ovate, 
narrowing anteriorly ; cardinal margin descending forward, and 
meeting the anterior and posterior margins at more or less 
defined angles; cardinal area extremely narrow, with about two 
deep, divaricating cartilage-furrows; lateral denticles very 
oblique, those on the posterior side more elongated and striated ; 
