24 Tnms. Acad. Sci. of St. Louis. 



The left valve, which is the only one known, compressed- 

 convex, the greatest convexity at a point about two-thirds of 

 the distance from the basal margin to the beak. The ears 

 triangular, the anterior one smaller and more abruptly flattened 

 from the body of the shell, its anterior margin nearly straight 

 or slightly convex, meeting the straightened lateral margin of 

 the shell below at an angle of about 150°; the posterior 

 margin of the posterior ear concave, forming with the lateral 

 margin of the shell below, a rather wide rounded or suban- 

 gular sinus. Beaks erect, extending slightly beyond the 

 hinge-line. Surface of the shell smooth. 



Remarks. A single specimen of this species has been 

 observed. In it the crenulated hinge-line of Crenipecten is 

 clearly seen, and in general form it resembles the specimens 

 referred to O. winchelli. It differs from those specimens, 

 however, in its lack of ornamentation and in the very differ- 

 ent form of the anterior ear. 



Pernopecten cooperensis (Shumard). 



Fl. IV. f. 22, 23. 



Shell compressed-lenticular, thin, nearly equivalve, sub- 

 orbicular exclusive of the ears, attaining a height of 

 34 mm. and a width of 32 mm. Basal margin rounded; 

 lateral margins rounded, sometimes slightly straightened above 

 and when continued to the beak meeting at an angle varying 

 in difi"erent specimens from 110° to 120°. Hinge-line very 

 short, one-third or less the greatest width of the shell. In 

 the left valve the hinge-margins slope slightly inward from 

 the extremities of the ears to the beak at the middle; in the 

 right valve the hinge-margin is straight. Ears small, flat, 

 triangular, equal or nearly so, obtusely angular at the ex- 

 tremities, with no trace of a byssal sinus on either side. 

 Beaks small, compressed, not projecting above the hinge-line 

 or showing the slightest obliquity. Superior lateral regions 

 of the valves compressed and separated from the more convex 

 central portion by shallow undefined impressions that diverge 

 from the beak at an angle of from 70° to 80°, the one on the 

 posterior side being usually the longer. The portion of the 

 shell between these compressed regions has an elongated 

 triangular form. 



