Shell normally flat, oblong, solid, with 10 stout rounded ribs projecting at the margins, of which 2 are in the axis of length with 4 on each side: ribs and interstices radiately striated: yellowish white, generally with more or less of black or brown tortoise-shell markings within, sometimes with the black between the ribs as described by Philippi, and Reeve. Sometimes the shell is more rounded and the ribs rather angular, in which state it might be taken for the young of P. mexicana. Occasionally a few other intercalary ribs appear. In a very few unusually large specimens, the ribs are nearly obsolete at the margin and the shell is much lengthened. The body mark varies as usual; when plain, it is gathered into points as in P. discors. The very young shells appear not to develop the ribs marginally, in which state they might be taken for the young of P. discors. The stout ribs of the adult shell however bear no analogy with the very finely marked surface of the latter with its curiously puckered circum-umbonal portion. With the young of P. Mexicana it has much closer analogies. The largest specimens of P. pediculus however do not at all run into the smallest of P. Mexicana. They have all the appearance of being old shells, with the margin narrow and the shape long and irregular; while P. Mexicana, as it is traced upwards, displays a very wide semitranslucent margin, and a broad regular shape, with the ribs not rounded and prominent but simply giving an angular form to the shell. To settle the point satisfactorily will require a series of P. mexicana from its earliest stage, as well as a knowledge of the animals. Of this species I have carefully examined nearly 300 specimens. The youngest shell that has its ribs developed and clearly belongs to this species is 0,17 inch long. Tablet 912 contains two smaller ones without distinct ribs, which probably belong to it. Even when very young, they are almost always incrusted with corallinous matter. The largest specimen measures long. 1,3, lat. 1inch, alt. 0,4. The ordinary size however is very much smaller and flatter.
Hab.—Mazatlan, Philippi: Acapulco, Reeve: Mazatlan; not uncommon