Circe rugifera
Similar to Circe intermedia. Circe rugifera is distinguishable by its more ovate, less trigonal shape and the wider angle between the anterior and posterior teeth at the hinge plate.
The overall shape is ovate. The anterior dorsal margin is slightly concave, and the posterior dorsal margin is slightly convex. The valves are equal in size and shape (equivalve), and there is no gape when they are closed. Sculpture consists of growth lines and comarginal ribs that become stronger 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 half the anterior dorsal margin. The umbo is central, and the beaks point slightly to the anterior. The escutcheon is lightly impressed, but otherwise not well defined.
The exterior may be white or light tan, and there may be light brown, extremely fine tent patterns on the flank or concentrated at the escutcheon and/or lunule. There may also be brown coloration along the comarginal ribs. The interior is white with a light brown-pink blotch in the center and/or concentrated at the posterior margin.
The ligament is external, although it is slightly sunken; it extends 1/4 of the posterior dorsal margin and is supported by nymphal ridges. On the left valve, there are three cardinal teeth. The anterior (2a) and posterior (4b) teeth are not bifid, and the central tooth (2b) is lightly 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, A3) on the right valve. A very fine 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.