Shell large for the genus, thin, conical, basal outline elongate-oval, anterior end slightly narrower than posterior, basal margin lying flat, with ends slightly elevated. Lateral slopes straight, anterior and posterior slopes slightly concave. Foramen nearly central, oval, length of foramen about 10% of shell length, highest point of shell at the posterior rim of the foramen. Periostracum thin, brown, worn away over most of the surface; underlying surface chalky, worn near summit. Sculpture of numerous fine radial ribs; under magnification, the ribs are rounded, broader than the interspaces, crossed by raised growth lamellae to give a minute beaded appearance; primary ribs remain stronger so that every fourth rib is slightly more prominent than the others. Interior chalky white, the margin finely crenulated and grooved by the ribs; position of the stronger primary ribs marked by corresponding internal grooves. Under magnification, the crossed-lamellar aragonitic structure near the margin is visible. Muscle scar weakly discernible. Callus surrounding the foramen has an oval outline and is slightly truncated and depressed at its posterior edge. Thickness of shell at summit (depth of foramen) approximately equal to the width of the internal callus.
Dimensions: length 40.7 mm, width 27.0 mm, height 11.4 mm (holotype); length 40.6 mm, width 26.9 mm. height 13.7 mm.
Animal of preserved specimen: colorless except for darkly pigmented eyes; contracted to fit completely within the shell; uppermost fold of mantle with a very finely scalloped edge, surface of the mantle folds and the foot sides with negligible development of papillae; cephalic tentacles and epipodial tenta¬cles well developed.

A species of Diodora characterized by its large size, thin shell, oval aperture, and numerous radial ribs. It most resembles D. tanneri (Verrill, 1883), known from archibenthal depths in the western North Atlantic, from Delaware to the Caribbean (see Perez-Farfante. 1943:19). D. codoceoae differs in having a less conical profile, a proportionately larger foramen, and a more chalky shell surface. We have no information about the animal of D. tanneri. Diodora codoceoae also resembles Stromboli beebei (Hertlein and Strong, 1951), an offshore species in the tropical Panamic Faunal Province ranging from the Gulf of California to Ecuador (sec McLean in Keen, 1971:318, fig. 29). Unlike D. codoceoae, S. beebei has a fleshy mantle that fully envelops the edge of the shell and radial ribs that arc uniformly fine, with no distinction between primary and secondary ribs; in addition. S. beebei has a sturdier shell than that of D. codoceoae.
The internal callus surrounding the foramen is curved on the posterior side in D. codoceoae. D. tanneri, and S. beebei, not straight-edged as in most species of Diodora. How¬ever, this is a variable feature among species of Diodora, particularly those with oval foramena.

Pichidangui (34°06'S) to Constitucion, Chile (35"20'S). Depth range 180-360 m.