36 



marked with distinct concentric lines of growth, and nu- 

 merous minute punctas. 



The extremely arcuate character of the larger valve of 

 this species, and the broad, regularly rounded sinus which 

 extends from beak to base, are very remarkable characters. 

 The smaller valve is sometimes nearly straight down the 

 middle, and in other specimens extremely gibbous. 



This species has a wide geographical distribution in the 

 west. The specimens described were received from E. Dan- 

 iels, Esq., who collected them at Topeka j another speci- 

 men which I designated by this name, is from the collec- 

 tions of the Pacific Rail Road Survey, under Lieut. Whip- 

 ple, and is marked with the locality of Pecos village. 



