5. Genus Phialopecten nov.
Type: Pecten triphooki Zittel.

Shell large, equilateral, right valve flat, left valve well inflated. Ears large, subequal; byssal notch narrow but well marked, no ctenolium. Sculpture: both valves with twenty to thirty strong, rounded ribs with narrow interstices. The ribs soon develop a weak central groove and the interstices a central thread, sometimes these increase in number until the whole surface is faintly corrugated.