374 NORTH AMERICAN INDEX FOSSILS. 



8 /// . Resilifer and ligament scar separated by a 



thin septum CLXXV. Mactra. 



S /// . Resilifer and ligament scar not separated by 



a septum t /// . 



e /// . Margins of the combined resilifer and 

 ligament pit elevated. 



CLXXVI. Cymbophora. 

 e fn '. Margins of pit not elevated. 



CLXXVII. Schizodesma. 

 q n '. Cardinal teeth three in each valve. 



CXLII. Corbie ula. 



xx. Shell semicircular and exceedingly thin CVI. Placunopsis. 



xx. Shell subcircular 25f. 



25-f-. Shell exceedingly thin, fragile, translucent 26*. 



26*. Lower valve flat, perforated for the byssus ; upper valve 



convex. CV. Anomia. 



26*. Lower valve flat, not perforated for the byssus. 



CVI. Placunopsis. 



25-f. Shell of median thickness 27*. 



27*. Beaks prominent -.24?'. 



24". Beaks spirally enrolled forward CLIX. hocardia. 



24". Beaks merely curved forward r". 



r f/ . Cardinal teeth three in each valve... q ff/ . 



9 7// . Pallial sinus present ■/ /// - 



/ 7// . Pallial sinus long CLXI. dementia. 



f". Pallial sinus short 4). 



4). Lateral teeth present b). 



b). A posterior lateral tooth in right 

 valve ; height and length of shell 



subequal CLXI V. Dosiniopsis. 



b). No posterior lateral present; length 

 of shell greater than height. 



CLXI II. Meretrix. 



4). Lateral teeth absent c). 



c). Inner margins of valves crenulate. 



CLXIL Venus, 

 c). Inner margins of valves smooth. 



CLXV. Tapes. 



</". Pallial line entire.. CXXXV. Arctica. 



; // . Cardinal teeth two in each valve io f// . 



io f " '. Left anterior and right posterior cardinal 

 teeth prolonged and curved. .CLVI. lenea. 

 io' f/ . Cardinal teeth not prolonged. 



CLV. Diplodonta. 

 r" . Cardinal teeth two in right valve and three in left. 



CLX. Cyprimeria. 

 24" . Beaks not curved ; shell inflated. 



CXLIII. Sphceruitn. 



27*. Beaks small and subcentral 25". 



25" '. Concentric sculpture strong and irregular. 



CLII1. Paracyclas. 



