62 
MARINE SHELLS OF WEST COAST OF NORTH AMERICA 
vexly striated but with no perceptible byssal notch or ctenolium; the right 
valve with a few faint concentric undulations near the umbo; beyond that 
smooth, near the lower margin concave and appressed against the margin 
of the opposite valve; left valve more convex, equilateral, smooth, polished 
with a few faint irregular radial markings near the base. Interior 
polished, the ligament very small. Length of hinge line, 9.5; disk, 24; 
height, 22; max. diameter, 5 mm. (Dali.) 
Type in Ottawa. Type locality, Dolphin and Union Strait, Northwest 
Territories. 
Range. Dolphin and Union Strait, Arctic Ocean. 
Subgenus Propeamusium Gregorio, 1883. 
Pecten alaskensis Dali, 1871. 
Plate 12, fig. 3, and plate 38, fig. 6. 
Amer. Journ. Conch., 7:155; pi. 16, fig. 4. 
Shell nearly equilateral, inequivalve, flesh color, with a blush of 
salmon color on the umbo of the superior valve. Internally white, the 
salmon color showing through the valve. Shell suborbicular, barring the 
auricles, which are wide and prominent. Lower (or right) valve flattened, 
0.1 inch (2.5 millimeters) smaller than the upper one; sculpture of fine, 
close, equal, concentric ridges, sharply defined and separated by narrow 
noncanaliculated grooves. Valves covered with a fine velvety epidermis, 
ashy and very finely radiately striate. Surface of the valve, except for the 
ridges, smooth. Anterior auricle long, prominent, with a deep sinus. Pos¬ 
terior auricle small; both with strong elevated lines of growth, which 
rise into scales on the 8 or 9 fine ribs with which the anterior auricle is 
furnished. Hinge line straight, smooth. Inside of the valve polished, 
furnished with 21 rounded, radiating ribs, with traces of others inter¬ 
calated near the margin; nodulous or swollen at the more prominent 
ridges of growth and at the margin. Upper (or left) valve similar 
inside; anterior auricle shorter, not so deeply sinuated. Valve more convex 
than the under one, and a little longer. Dorsal areas finely granulate. 
Umbo smooth; half way toward the margin the striae increase, become 
more conspicuous, and about 35 pseudoribs radiate toward the margin. 
These are formed by the elevation of the concentric lines of growth like 
ruffles, in such a way that the edge of one fluting of the ruffle overhangs 
the beginning of the next, and so on. These are very fragile, and when 
broken away show the nearly smooth surface of the valve underneath, 
without any true rib at all. Faint ribs are intercalated between the 
pseudoribs toward the margin. Height, 19.5; breadth, 19.5; diameter, 
5 mm. (Dali.) 
Type in U. S. N. M. Type locality, Pribiloff Islands, Alaska. 
