Arca reinharti: Guaymas, 20 fathoms (1932). Type 11389, Lowe collection.
Shell ivory white, covered with a brown, horny epidermis; obliquely rhomboid, solid, equivalve; edges of valves thick; anterior end rounded, posterior end angularly extended downward; about 25 radiating ribs, with narrow interspaces; the ribs toward the anterior end are strongly nodulous. The shell somewhat resembles a miniature specimen of A. multicostata Sby., but is much more oblique, has about ten less ribs, and has a ligamental area (in specimens of equal size) of three times the diameter. In young A. multicostata the left valve overlaps the right.
In young A. multicostata the edges of the valves are quite thin, while those of the species under discussion are abnormally thickened; the grooves on the inner margin of the valves extend almost four times as far within the shell.
To compare the new species with one of almost the same size of A. multi¬costata the following measurements are given: Arca reinharti—diameter 27.7 mm., altitude 22.1 mm., thickness 24.5 mm.