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VENERIDAE 405
1% inches in length, externahy very much hke a small Mercenaria
campechiensis, but not as elongate and with more distinct, concentric riblets.
Internally, it is distinguished easily by the extremely small, if not absent,
pallial sinus, by the very thick margin of the shell, and in the left valve by
the presence of a button-like anterior lateral "tooth." Exterior whitish.
Dredged occasionally from 40 to 70 fathoms. Considered a collector's item.
Figure 8 1 . Some
venerid clams, a and b, Co7npsomyax subdiaphmia Cpr., 2 inches
(Pacific Coast); c, Dosmia discus Reeve, 2 inches (Atlantic Coast); d, Aiitigo7ia
strigillina Dall, i^ inches (Florida and West Indies); e, left valve of Pitar
monhuana Linsley, 1% inches (Atlantic Coast).
Ant'igona rugatina Heilprin Queen Venus
North Carolina to southeast Florida and the West Indies.
Plates 38m; 3211
I to i^ inches in length, rather circular, inflated, and characterized by
strong, raised, lamellate, concentric ribs between which are 5 or 6 smaller,
raised concentric ridges. Lunule heart-shaped, well-impressed, bordered by
a fine, deep line, and crossed by numerous raised threads. Escutcheon well-
formed, smoothish. Color cream to whitish with light-mauve mottlings.
Very uncommon.
Antigona rigida Dillwyn from the West Indies is very similar, but
not nearly so obese, and its concentric ridges are stronger and smoother.
^^Chiojiid" Group
Ovate-trigonal, inequilateral, sculpture usually cancellate; lunule im-
pressed. Inner margins usually crenulate. Teeth strong, without the tiny,
pimple-like anterior lateral. Pallial sinus short. This section or group is