1 84 NORTH AMERICAN INDEX FOSSILS. 



4. Shell strophomenoid — Length and breadth subequal. Hinge line forming 

 usually the greatest width of the shell. Shell usually concavo-convex, 

 never strongly biconvex. Cardinal areas well developed ; delthyriura 

 triangular and usually closed (open in Vitulina, Tropic! oleptus, Syntro- 

 phia, Scenidium). Median sinus and fold usually absent ; when pres- 

 ent very broad and ill-defined. Surface radially striate, seldom plicate 



or smooth k. 



k. Surface radially striate 555. 



555. Margins of hinge line finely grooved transversely qq. 



qq. Shell normally concavo-convex (brachial valve concave, 



pedicle valve convex) XXXII. Stropheodonta. 



qq. Shell resupinate (brachial valve convex, pedicle valve con- 

 cave) XXXIV. Strophonella. 



555. Margins of hinge line not transversely grooved rr. 



rr. Cardinal margin of pedicle valve with a simple row of 



spines i6f. 



l6f Shell normally concavo-convex 15*. 



15* Surface simply striate radially... XLII. Chonetes. 

 15* Surface with a double oblique series of striae ex- 

 tending from umbo XLIV. Chonopectus. 



l6f Shell resupinate XLI1I. Uionostrophia. 



rr. Cardinal margins without spines I7f. 



I7f Radiating striae alternating in height 16*. 



16* Shell normally concavo-convex I 5 // - 



15" Shell large XXXI. Rafinesquina. 



15" Shell small XXXVII. Plectambonites. 



16* Shell resupinate XXXV. Strophomena. 



17-f- Radiating striae subequal in height 17*. 



17* Front of shell geniculate (abruptly bent almost at 

 right angles) ; concentric wrinkles on older 



portion XXXVI. Lepl&na. 



17* Front of shell not geniculate 16". 



16" Radiating striae prominent, sharp, crowded ; 



umbo apt to be distorted e N . 



e" Pedicle valve with spondylium. 



LIX. Clitambonites. 

 e ff Pedicle valve without spondylium... 4 /// . 

 4 7// Muscular area of pedicle valve sur- 

 rounded by a strong ridge. 



X L I . Hipp a rionyx. 

 4 /// Muscular area of pedicle valve not 

 surrounded by a strong ridge. 



b'". 

 b fff High median septum present, 

 running through center of 

 muscular area. 



XXXIX. Orthothdes. 

 b /;/ High median septum not pres- 

 ent.. XXXVIII. Schuchertella. 



id" Radiating striae faint f". 



f ff Shell concavo-convex, resupinate. 



XXXV. Strophomena. 



