PELECYPODS 425 
A. aculeata is much smaller than the last species. 
and the surface of the convex valve is roughened AWS: 
by prickly scales. The smaller valve is very thin, § 
ae almost cas i pan oa the byssus. Es38 y 
ts diameter is about half an inch. Found at- ; ; 1 
tached to stones, ete., about the holdfasts of Fucus. “inomiacsuteata from below, 
A. lampe, a California species ranging more 
to the south, is small (half an inch), and greatly resembles the east- 
coast A. aculeata, save as regards the smooth surface of the former. 
GENUS Placunanomia 
P. macrochisma. This species occurs on the Californian coast. It has 
the same sort of calcified byssus as Anomia, and also the hole to accom- 
modate it in the smaller valve. The scar formed by the muscle which 
controls the byssus is curiously rayed in this genus. The normal shape 
of the shell is circular; it is pearly within, and of a greenish tinge with- 
out. Two to four inches in diameter. (Plate LXXVIII.) 
FAMILY ARCIDE 
GENUS Arca 
The Arcide may always be distinguished by the long row of 
comb-like teeth upon the hinge, the total want of siphons (a fact 
which may be discovered in the shell alone by its lack of a pallial 
sinus), and the solid trapezoidal or round shell, with its tendency to 
strong ribs radiating from the umbones. The periostracum, or 
epidermis, is heavy and often velvety or even hairy. The animal 
of Arca has the.gills placed in an oblique position; the foot is 
large and strongly developed. No trace of mantle fusion exists. 
The mantle-edges bear a row of composite eyes. The gill-fila- 
ments are entirely free, there being even no ciliary junctions. 
A. pexata. An _ exceedingly 
common species in Long Island 
Sound and on the coast of New 
Jersey. The shell is oblong, with. 
prominent beaks directed forward. 
The hinge-teeth are arranged in 
arow posterior to the beaks, while 
just under the beaks are a num- 
ber of irregular cartilage-pits. 
About thirty-two to thirty-six ra- 
diating ribs ornament the shell. 
The inner margins of the shell are 
rather deeply scalloped; the epi- 
Arca pecata, dermis is thick, shaggy, and dark 
