PELECYPODA—PRIONODESMACEA. 507 



A characteristic Coal Measures species : Ohio to Oklahoma and 

 Wyoming ; also in Mississippic of Ohio and Arizona. 

 393. P. (Entolium) operculiformis Gabb. (Fig. 679.) 



Comanchic-Cretacic. 



Height greater than breadth. Sides of umbo straight, forming 



Fig. 678. Entolium aviculatum, left valve. (Ind. Surv. ) 

 Fig. 679. Entolium operculiformis. (Pal. Cal.) 



almost a right angle. Surface smooth, polished. 



Chico : California. Horsetown : California, Oregon. 



CI. Syncyclonema Meek. 



A small, nearly equivalve, vertically ovate Pecten. Hinge line 

 very short. Ears very small, the anterior the larger. Margins 

 closed all around, with no defined byssal sinus in either valve. 

 Surface only concentrically striated. Cretacic. 

 394. Pecten (Syncyclonema) rigidus Hall and Meek. Cretacic. 



Hinge line about half the transverse diameter of the valves. 

 Posterior ear obliquely truncate, and hardly half as large as the 

 anterior which is subtrigonal, and slightly pointed. Right valve 

 marked with rather strong and regular concentric ridges and fur- 

 rows ; left valve smooth. Anterior margin with only a shallow, 

 rounded sinuosity under the ears which is nearly as deep in the 

 left valve as in the right. Average length 0.19 inch ; height 0.23 

 inch. Differs from P. simplicins in the concentric ridges of the 

 right valve, the more regularly rounded outline of its narrower 

 valves, and in the different form of its ears. 



Montanan : South Dakota, Montana, Wyoming. 



CII. Amusium Schumacher. 

 A Pecten with raised radial riblets internally. Smooth exter- 

 nally or faintly striated. Valves of about equal convexity, large, 



