590 NORTH AMERICAN INDEX FOSSILS. 



9*. Whorls few, rapidly enlarging n. 



n. More or less globular except when uncoiling 99. 



99. Inner lip thickened by callus ii. 



ii. Inner lip flattened, callus smooth or transversely striate; 



aperture oval LXVIII. Naticopsis. 



ii. Inner lip not flattened, callus smooth, aperture semi- 

 circular to oval CXIII. Natica. 



ii. Callus with straight, commonly denticulate border. 



XCVI. Nerita. 



99. Inner lip not thickened by callus jj. 



jj. Lower lip twisted and grooved, aperture round. 



LXXII. Strophostylus. 

 jj. Inner lip with columellar fold, aperture elongate. 



LXXXIX. Sphcerodoma. 

 n. More or less conical, with base flat, and last whorl extended so 



as to form flat periphery CVIII. Calyptraa. 



n. Elongate, body whorl extended 000. 



000. Coiling dextral or right-handed kk. 



kk. Shell thin, body whorl moderate CCXI. Limncea. 



kk. Shell thick, body whorl very large. 



CXCIX. ActcBonia. 



000. Coiling left-handed or sinistral CCXII. Physa. 



9*. Whorls regularly and gradually enlarging o. 



o. Spire low, columella not plicate III. 



ill. Close-coiled 11. 



11. Large 6f. 



6f . Whorls few, aperture circular. 



LXXIII. Diaphorostoma. 



6f . Whorls many, aperture with umbilical extension. 



LXVI. Pycno?nphalus. 



11. Medium sized, aperture interrupted at body whorl ; 



outer lip frequently with varix CCXIX. Helix. 



11. Minute 7f. 



7f. Vertically compressed LXV. Anomphalus . 



7f. Last whorl ventricose LXVA. Proto spiralis. 



ill. Last portion not coiled ; aperture circular or variable. 



LXXIV. Platyceras. 



o. Spire low, columella with plications 222. 



222. Shell broadest in upper part, often with distinct shoulder 



angle mm. 



mm. Surface smooth CCVIII. Melampus. 



mm. Surface with wrinkles or faint ribs in upper part. 



CCIX. Rhytophorus. 

 222. Shell broadest in middle or below, no shoulder. 



CCX. Alexia. 



o. Spire moderately high ZZZ- 



333. Aperture circular or subcircular nn. 



nn. Aperture generally more than half the height of shell. 



LXXI. Holopea. 

 nn. Aperture generally less than half the height of shell. 



8f. 



