372 NORTH AMERICAN INDEX FOSSILS. 



i8 // . Byssal opening very prominent..LI. Lunulicardium. 



18". Byssal opening not prominent CXVI. Mytilus. 



21*. Beaks small, appressed, very anterior but not terminal. 

 Shell rounded anteriorly and posteriorly, rather tumid, 



modioliform i</ f . 



jg n ' . Cardinal area with many longitudinal grooves for 



ligament m // . 



m // . Grooved area on both sides of the beaks. 



LXXVI. Ptychodesma. 

 m // . Grooved area only posterior to beaks. 



CXI II. Eurymya. 



/p // . Cardinal area not grooved n ,f . 



n". Teeth present ^ /// . 



4 f// . Cardinals one to three in each valve a /// . 



a /// . Surface with strong concentric ridges. 



CXXXIII. Cypricardinia. 



a /// . Surface with concentric striae /). 



/). Anterior adductor very deeply impressed. 

 XLII. Ischyrodonta. 

 /). Anterior adductor not very strongly im- 

 pressed CX. Modiolodon. 



4'" . Cardinals, one strong in left valve and corre- 

 sponding cavity in right. 



CIX. Modiomorpha. 

 4 fn ' . Cardinals, one strong in right valve and cor- 

 responding cavity in left... CXI. Colpomya. 

 4 fn '. Teeth few, microscopic, beneath the beaks. 



CXVI I. Modiola. 



n". Teeth absent j'". 



5 /// . Furrow present, passing from in front of the 



umbo to the posterior basal margin b fn '. 



b fn '. Umbonal ridge strong and angular. 



CXV. Goniophora. 



b /;/ . Umbonal ridge rounded or wanting 2). 



2). Umbonal slope sharply limited anteriorly. 



a). 



a). Umbo long and pointed 



CXVII. Modiola. 

 a). Umbo broad and rounded. 



LXXVII. Modiella. 

 2). Umbonal slope not sharply limited an- 

 teriorly C V 1 1 1 . Modiolopsis. 



j /// . No furrow present c /// . 



/// 



Valves equal CXII. Whiteavesia. 



c ,n '. Valves unequal ; shell minute. 



CXIV. Aristerella. 



xx. Shell broadly oval to quadrate 23-f. 



23-f. Beaks prominent 22*. 



22*. Umbonal ridge prominent CXXXVI. Veniella. 



22*. Umbonal ridge wanting 20". 



20 n '. Posterior cardinal tooth in right valve bifid at its 



