588 NORTH AMERICAN INDEX FOSSILS. 



dd. Surface smooth XIX. Bellerophon. 



5*. Umbilicus minute or absent g. 



g. Aperture abruptly expanding 55. 



55. Lip sinuate, no slit-band XXII. Ptomatis. 



55. With narrow strong slit-band XXIII. Phragmostoma. 



g. Aperture not abruptly, but regularly expanding 66. 



66. Aperture with central emargination and slit-band, often on 



median keel ee. 



ee. Inner lip expanded as a flat septum. 



XXIV. Carina?- opsis. 



ee. Inner lip callous, but not expanded into septum 4-f. 



4f. Surface with growth lines only ; umbilicus small 



or absent XIX. Bellerophon. 



4f . Surface with revolving as well as growth lines \" . 



l /; . Revolving stride uniform over entire shell. 



XXI. Bucanopsis. 

 l ff . Striae in form of parallel ridges on inner lip 

 and over part of preceding volution. 



XX. Euphemus. 

 66. Aperture without slit-band, but deeply bilobed ; surface 

 finely spiraled (or smooth), umbilicus mostly small. 



XI. Protowarthia. 

 4. Shell with rounded dorsum, modified by revolving keels or folds or by 



nodes 6*. 



6*. With revolving folds only h. 



h. With median fold, bearing short slit, and one or more lateral 



folds on each side XIII. Tetranota. 



h. Dorsum deeply trilobed ; aperture sinuate XII. Bucaniella. 



6*. With nodes or transverse ribs i. 



i. Umbilicus large XXV. Porcellia. 



i. Umbilicus small or absent XIX. Bellerophon. 



4. Shell with sharp dorsal keel, strongly compressed laterally 7*. 



7*. Whorls more or less triangular j. 



j. Sides angular, generally with ribs, slit short or absent. 



IX. Cyrtolites. 

 j. Sides rounded, strong lamellose growth-lines ; slit long. 



XVIII. Phragmolites. 

 7*. Sides flatly rounded, shell disciform, strongly compressed keel 



sharp XVII. Oxy discus. 



II. Inner whorls forming low spiral, outer whorl in same plane 5- 



5. With median notch and slit-band ; shell marked by nodes or ribs. 



XXV. Porcellia. 



5. Without median notch or slit, surface smooth CCXIV. Planorbis. 



II. All whorls very nearly in same plane 6. 



6. Rapidly enlarging ; shell minute CCXX. Pelagiella. 



6. Gradually enlarging CCXIV. Planorbis. 



C. Shells naticoid or varying from flat to oval, with round whorls showing rarely a 



faint angulation and forming a moderate sometimes flat spiral. Final 

 (sometimes all) whorls may be loose-coiled ...III. 



III. Surface without ribs or spirals, but growth-lines may be prominent 7. 



7. Shell umbilicated 8* 



