30 NORTH AMERICAN INDEX FOSSILS. 



77. Whorls with pronounced angulation, but not annulated or noded 



in adult dd. 



dd. Venter channeled in adult 5|. 



5t. With two pairs of revolving ridges... XXXI. Stroboccras. 



5t. Without the revolving ridges. .f. 



f. Suture with ventral and lateral lobes and broad dorsal 



saddle XXXII. Apheleceras. 



f. Suture with ventral subacute saddle and broad lateral 



lobes XXXIII. Ephippiocems. 



dd. Venter convex in adult, often channeled in young Gf. 



6f. Section triangular, whorls striate... XXXIV. Triboloceras. 



6f. Whorls elliptical, not striate XLVIII. Edaphoceras. 



d. Surface with annulations 88. 



88. Shell close coiled ; annulations fine, regular and crowded. 



LI I. Cymatoceras. 

 88. Shell widely umbilicated, gyroceraconic, or whorls slightly im- 

 pressed ee. 



ee. Discoidal whorls very gradually enlarging, costse sharp, 



distinct XXIII. Discoceras. 



ee. Whorls regularly enlarging ; costse low, crowded. 



XXIV. Plectoceras. 

 Z^. Shell a trochoceran spire XXV. Sphyradoceras. 



d. Surface of shell with two or more rows of nodes 99. 



99. Whorls laterally compressed, higher than wide, with nodes on the 



ventrolateral margin ff. 



ff. Sides flat or concave, section subquadrangular 7"f". 



7f. With ventrolateral rows of tubercles only (often obsolete). 



XLII. Metacoceras. 

 7f . With double row of lateral as well as ventral tubercles. 



XLIII. Tainoceras. 

 ff. Sides convex, height much exceeding width. 



XXXIX. Centroceras. 



99. Whorls as wide as high or vertically compressed gg. 



gg. Venter gently arched, flat or with concavity, tubercles on 



ventrolateral angle, often flat spines XL. Teninocheilus , 



gg. Venter strongly arched, tubercles near umbilicus or obsolete. 



XLI. Endolobus. 

 **. Trochoceracones, coiled in more or less pronounced spiral e. 



e. Surface smooth 000. 



000. Spire high, whorls regularly enlarging. Coil right-handed. 



XXVI. Mitroceras. 

 000. Spire low, coiling, left-handed LVII. Ncedyceras. 



e. Surface annulated , spire low XXV. Sphyradoceras. 



I. Cyrtochoanites : Straight, curved or coiled shells, siphuncle generally highly 



nummuloidal, funnels bent outward or crumpled, short ***. 



■5t-5f*_ Without contracted aperture ...f. 



f. Siphuncle mostly without deposits ill. 



III. Orthoceracones LVI. Lcxoceras. 



III. Trochoceracones LVII. A^cedyceras. 



III. Gyroceracones LVIII. Gigantoceras. 



