CLASS PELECYPODA 
47 
layer is entire. The type is P. nigra Ch. and it is represented in our recent 
fauna by P. rigida Dillwyn and P. serrata Sowerby. (Dali, 1895.) 
Type. Pinna nigra Ch. 
Atrina oldroydi Dali, 1902. 
Plate 28, fig. 12. 
The Nautilus, 14:143. U . S. N. M. Bull., 112; pi. 2, figs. 4, 5, 6. 
Shell solid, heavy, blackish gray, subtriangular, rather inflated; um- 
bonal end slender (somewhat defective in the specimen) ; hinge margin 
straight; ventral margin contracted in front, convexly arcuate behind; 
posterior margin arched; exterior smooth, except for more or less con¬ 
centric wrinkling on the ventral side and numerous rather fine imbricate 
elevated ridges (about 38) radiating from near the umbo on the dorsal 
and middle portions of the valve, not extending to the ventral surface and 
obsolete over the distal valve; the scales or spines are worn ofif, but appear 
to have been numerous and small; interior of a livid dark olive gray, with 
a lurid iridescence over the visceral area, the ventral edge of which extends 
in a zigzag line almost directly anterior from the ventral edge of the rather 
small adductor scar, leaving more than a third of the ventral surface of the 
valve exterior to the visceral area. Length of ventral margin, 238; of 
dorsal margin, 175; of the distal margin, 156; maximum diameter of the 
valves, 63 mm. Length of the visceral area from the umbo, 172 mm. The 
byssus is quite small and of a dark blackish brown color. The form of the 
visceral area, which in these shells is generally regarded as a pretty con¬ 
stant character, is entirely different from that of any of the other described 
Pacific coast species. (Dali.) 
Type in the Oldroyd collection, Stanford University, No. 117. Type 
locality, off San Pedro, California, in 25 fathoms. 
Range. San Pedro Bay, California, in deep water. 
Family PTERIIDyE 
Genus PTERIA Scopoli, 1777. 
Shell obliquely oval, very inequivalve, eared, the posterior ear pro¬ 
duced, wing-like; right valve with a byssal sinus beneath the anterior ear; 
cartilage pit single, oblique; hinge with one or two small cardinal teeth, 
and an elongated posterior tooth, often obsolete; posterior muscular im¬ 
pression (adductor and pedal) large, subcentral; anterior (pedal scar) 
small, umbonal. = Avicula Lamarck, 1799. (Tryon. S. S. Conch.) 
Type. Mytilus hirundo L. 
Distribution. Mexico, South Britain, Mediterranean, India, Pacific. 
