574 A. FE. Verriil— Catalogue of Marine Mollusca. 
Variety, Frielei (Jeffreys) Verrill. 
Arca frielei (Jeff. MSS.) Friele, Mag. Naturvid., xxiii, p. 2, 1877. 
Jeffreys, Proc. Zool. Soc., London, 1879, p. 573, pl. 45, figs. 4, 4a. 
PLATE XLIV, FIGURES 5, 6. 
This shell, as it occurs on our coast, is exceedingly variable in 
form and size. Our largest specimens are about 15™" long. In 
outline it varies from an elongated oblong form, like fig. 5 (variety 
grandis) to a short-oblong (fig. 6); and from forms in which the 
anterior and posterior ends are nearly equally broad, to those in 
which the anterior end is very much narrowed, when the ventral 
edge becomes very oblique, and sometimes incurved, at the byssal 
notch. This elongated form, contracted anteriorly, is variety septen- 
trionalis. It is one of the most abundant forms at most of the sta- 
tions south of Martha’s Vineyard, though forms intermediate, in 
various degrees, between this and the oblong-form (var. grandis) are 
equally abundant. 
Another extreme variety is both short and oblique, combining the 
contraction of the anterior end with the shortening of the shell, so 
that the length is not greater than the distance from the dorsal to 
the ventral edge. In this form, the teeth are usually few, faint, and 
very oblique, confined to near the ends of the hinge-margin, and not 
uncommonly the teeth are nearly obsolete, and the shell is thinner 
and more delicate than usual, but the inner margin, as in the other 
varieties, is not crenulated. This form I regard as variety /rielez. 
It passes by insensible gradations into the variety septentrionalis. 
In all these varieties the inner margin of the shell is plain, and the 
hinge-teeth are usually few and oblique, especially the posterior ones, 
which are more or less lamellose, sometimes running almost parallel 
with the hinge-plate. The median portion of the hinge-plate is des- 
titute of teeth. The valves are more or less unequal in all the 
varieties, and are always finely decussated by slender, raised radiat- 
ing ribs and concentric lines, while the epidermis is always, in fresh, 
living specimens, more or less pilose, the hairs arranged in radiating 
rows, along the ribs. 
This shell attaches itself to pebbles or gravel-stones by a small, 
but strong, ventral byssus. 
This species has been taken in numerous localities by the various 
dredging parties of the United States Fish Commission, since 1872: 
in the Bay of Fundy, 108 fathoms; off Casco Bay, 94 fathoms; 
Gulf of Maine, 110 to 150 fathoms; Cashe’s Ledge, 27 to 90 
