546 



NORTH AMERICAN INDEX FOSSILS. 



Aquia : Maryland. Tejon : California. Eocenic of Alaska. 



CXLV. Chama Linne. 

 Unequal, irregular, attached by umbo of left and larger valve. 

 Beaks more or less spiral, turned forward. Hinge plate heavy. 

 Hinge teeth one in the free valve ; two in the attached, the an- 

 terior broad and grooved, the posterior long and curved parallel 

 with the hinge line. Sculpture lamellose and more or less spiny. 

 Adductor impressions large, oblong, nearly equal. Ligament and 

 resilium external in a deep groove. Shell structure in three layers, 

 the external colored, laminated with oblique lines of growth and 

 corrugated at right angles to the laminae, spiny ; the middle layer 

 corrugated ; the inner translucent and bearing minute processes 

 that give a granular appearance to the internal molds of the shell. 

 Sometimes attached by right valve, in which case the dentition is 

 reversed, the single tooth being always in the free valve. Cretacic- 

 Recent. 

 514. C. congregata Conrad, (pig. 748.) Miocenic-Recent. 



FlG. 748. Chama congregata, interior of left and exterior of right valves. 

 (Md. Surv.) 



Small, plump, usually attached by beak of left valve which is 

 quite turbinate. Internal margins crenulated. Both valves cov- 

 ered with scales. 



Miocenic : Atlantic coast. Also Recent : North Carolina to 

 Yucatan. 



CXLVI. Requienia Matheron. 



Usually very inequivalve, attached by umbo of left valve. Left 

 valve spiral, with deep cavity. Right valve smaller, somewhat 

 spiral or flat. Teeth feeble. Ligament external. Posterior ad- 

 ductor scar bordered by a prominent, subspiral ridge in each valve. 

 Comanchic. 



