Circe intermedia Reeve, 1863

The overall shape is ovate. The anterior dorsal margin is subconcave and the posterior dorsal margin is subconvex. The posterior half of the flank is slightly more inflated than the anterior half. The valves are equal in shape and size (equivalve), and there is no gape between them. Sculpture consists of growth lines and comarginal ribs that become more prominent as the animal grows. There are also oblique ribs: the anterior and posterior oblique ribs are restricted to the nepioconch, where they meet and form a wave pattern. The lunule is lightly impressed, defined by a fine shallow groove and extends 1/2 of the anterior dorsal margin. The umbo is slightly anterior and the beaks also point slightly to the anterior. The escutcheon is slightly smoother due to its lack of comarginal ribs, but is otherwise not well defined.

The exterior color is white or light tan and there may be brown tent marks or blotches. The lunule and escutcheon are typically marked with brown or purple stripes. The interior is white. There may be purple pigmentation in the central area of the interior, or concentrated at the posterior.

The ligament is external, extends 1/4 of the posterior dorsal margin and is supported by nymphal ridges. On the left valve, there are three cardinal teeth (2a, 2b, 4b), none of which are bifid; the posterior tooth (4b) is fused with the nymph. There is also one anterior lateral tooth (A2) on the left valve. On the right valve, there are three cardinal teeth (3a, 1, 3b), none of which are bifid. There are also two anterior lateral teeth (A1, A2) on the right valve. A weak ridge runs parallel to the posterior dorsal margin on both valves. There are fine antimarginal crenulations on the ventral and anterior margins. The anterior and posterior adductor muscle scars are similar in area, but the anterior is slightly more elongate. The pallial sinus is absent or extremely short.