106 



or feature, never affeotin.p- all the ri"bs. This species is reeoprni- 

 zed "bj' its '.veil inflated, si^-borhicular outline and numerous strong, 

 subroiinded rat^ials. It is closelj' relatea to P. vaginulus "biit is 

 slightly larger, more inflatei, with the right valve inflated to 

 the same degree as the left, more rou'de'^, itB rihs are hroader 

 and never as flat as those of many soeaiens of vaginulus. Al- 

 though end members of the two series approach each other in oxit- 

 line or in the degree of inflation, the^; may he separated hj? one 

 or more of the above 'differences. I-imature individuals of the two 

 species resemble each other very closely, but usually the ensem.ble 

 ■ the charaoters is surficient to differentiate them. 



Ocourren/. .- lower I.Iiocene: Bov^den beds, Bo^vden, Jamaica. 



