Shell of the general form of P. asper Sby., flattish, oblique, both valves similarly sculptured ; left valve with about eighteen ribs with nearly equally wide interspaces ; these ribs have three sharp thin keels upon them, a median and two lateral ones, which project on the distal margin of the valve, and, with the similarly projecting points of four intercalary smaller keels, fimbriate the margin in a remarkable manner ; when perfect there are rounded scallops thrown from keel to keel close together, like the edges of a roll of coins, which hide all the keels except the extreme edge of the median keel of the ribs, which, without projecting much, forms a raphe, connecting the scalloped surface ; the material of the scallops is very fragile, and when worn away leaves a totally different surface, which in this case shows a multitude of sharp thin scales (the bases of the scallops) zigzagging from keel to keel and imbricating the keels ; the ears are moderate and subequal, there are about five distant narrow ridges on the anterior and two or three on the posterior ear of the left valve, with finer intercalary threads ; the right valve has the ridges on the posterior ear scaly, and the byssal fasciole transversely concavely ridged; there are four or five free spines to the pectinium ; the cardinal margin is straight and internally strongly vertically striate, as in very young specimens of Janira or Amusium. Height of largest valve, 36.5 ; width, 36.5 ; cardinal margin, 19.0 mm. Umbonal angle about 100°.
Dredged living at Station 32, in 95 fins., north of Yucatan Banks, in N. Lat. 23b 32' and W. Lon. 88° 5'; bottom temperature not noted, but probably about 60° F.
Dead valves were found at Station 45, in 101 fms., bottom temperature 61°.75 F.; at a depth of 127 fms., off Havana, by Sigsbee; and off Grenada, at Station 244, in 792 fms. It is probably a dweller in about 10" fms. when living. The very remarkable sculpture of this species sufficiently distinguishes it from any other, and specimens not fully adult may be recognized at once by the fimbriated basal margin of the valves. It is related to P. Philippii Recluz and P. noronhensis Smith, differing from both in having more numerous ribs, flatter and similar valves (the two valves are differently sculptured in the others), and in details of form and sculpture of surface and ears.
In one specimen, apparently quite adult, though not as large as one of the dead valves, there are on the interior of the valves, especially the left one, narrow lira) corresponding to grooves bounding the ribs externally, but which are hidden on the outside by the imbricated sculpture. These lirae are very distinct, and are raised at the ends into a little white prominence like the lira) of A. alaskensis ; one more instance of the interchangeability of characters in this group.
The soft parts are streaked with purple in dots and dashes ; there is an ocellus for each rib on the margin, except near the anal opening, where the mantle margin is without them, and is folded in such a way as irresistibly to suggest that it is the first step toward siphonation; there are here two very peculiar large crimson color marks on the mantle edge not duplicated elsewhere. The ocelli are of different sizes, some much larger than others. The remainder of the superficial anatomy calls for no special remark.

Widely fan-shaped, posterior auricle only slightly smaller than anterior, compressed, solid, dull gray with indistinct pink blotches, with ca. 17 sharp, M-shaped radial ribs, each adorned with three rows of fine, closely packed scales; interior white with pink markings. Massachusetts to Florida, Bermuda, West Indies, Gulf of Mexico. Length 40 mm (to 42 mm).