Original description, by Lamarck (1818, p. 575):
"C. testá lentiformi, convexo-depressá, transversim sulcatá lineisque ferrugineis undatis pictá; natibus depressis; ligamento tecto.
Habite l'Océan des grande Indes. Mus. no Espèce rare, voisine de la suivante; mais qui en est très-distincte. Son ligament est caché et intérieur. Son bord antérieur est arqué jusqu'anx crochets. Le corselet et la lunule sont noirs, et très-étroits; largeur, 41 millimètres."
Similar to Circe plicatina. Circe undatina can be distinguished by its more ovate shape, more inflated valves and finer comarginal ribs.
Similar to Circe scripta. Circe undatina is more ovate in shape and its valves are more inflated.
The overall shape is ovate. The posterior margin is slightly truncated, and the anterior and ventral margins are rounded. The anterior and posterior dorsal margins are both nearly straight. The valves are equal in size and shape (equivalve) and not very inflated; 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 at the anterior and posterior of the nepioconch only. The intersection of the comarginal and oblique ribs creates fine nodules at the anterior and posterior areas of the nepioconch. The lunule is defined by a fine shallow groove and extends 1/3 of the anterior dorsal margin. The umbo is slightly anterior and the beaks point slightly to the anterior. The escutcheon does not have ribs, but is otherwise not well defined.
The exterior color is white or light tan with light brown blotches or tent marks that may form comarginal or radial patterns. The lunule and escutcheon are typicaly marked by dark brown pigmentation. The interior is white and may have brown and yellow coloration in the central area. There may also be brown coloration at the posterior dorsal margin.
The ligament is external, but slightly sunken beneath the surface of the flank; the ligament is short and extends only 1/5 of the posterior dorsal margin. On the left valve, there are three cardinal teeth. The anterior (2a) and posterior (4b) teeth are not bifid, and the center 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. The anterior (3a) and central (1) teeth are not bifid, and the posterior tooth (3b) may be lightly bifid. There are also two anterior lateral teeth (A1, A3) on the right valve. A fine ridge runs parallel to the posterior dorsal margin on both valves. The posterior adductor muscle scar is slightly larger and more elongate than the anterior adductor muscle scar. The pallial sinus is absent.