28 NORTH AMERICAN INDEX FOSSILS. 



I. Orthochoanites : Shells of varying form, straight, curved or coiled, with open 

 aperature, with relatively small siphuncle, mostly cylindrical, though 



sometimes more or less distended between septa (nummuloidal) **. 



**, Straight forms b. 



b. Shell annulated throughout 22. 



22. With longitudinal striae only in young X. Protocycloceras. 



22. With discontinuous longitudinal ridges throughout. 



XL Cycloceras. 



22. With continuous longitudinal strife XVI. Spyroceras. 



22. Without longitudinal ridges, but with frilled growth lines between 



the annuli XII. Dawsonoceras . 



b. Shell annulated in intermediate stages only. Longitudinal striae in 



young XIV. Kionoceras. 



b. Shell not annulated or only faintly so 33. 



33. Shell never strongly compressed bb. 



bb. Surface smooth VIII. Orthoceras. 



bb. With row of elongate tubercles on one sidc.IX. Trematoceras. 



bb. With longitudinal striations or ridges f|. 



f f . Longitudinal sculpture only in young, surface sometimes 



faintly annulated XIV. Kionoceras. 



If. Longitudinal sculpture throughout, fine and generally 



close XIII. Protokionoceras. 



33. Shell strongly compressed dorsoventrally. XLVII. Tripteroceras. 



b. Shell annulated internally (shown in mold), smooth exteriorly. 



XV. Orygoceras. 



**. Curved (cyrtoceraconic) or loose coiled (gyroceraconic) forms, the whorls 



not in contact c. 



c. Laterally compressed, section depressed elliptical 44. 



44. Cyrtoceracones with frilled growth lines, venter narrowest. 



XXVII. Zitteloceras. 

 44. Gyroceracones, venter widest....: XXVIII. Halloceras. 



c. Section subtrigonal or subcircular, whorls slightly compressed dorso- 

 ventrally 55. 



55. With coarse lamellose expansions at frequent intervals, sometimes 



spinous XXIX. Ryticeras. 



55. Without coarse expansions ; smooth' or noded. 



XXVIII. Halloceras. 



c. Section elliptical ; the whorls strongly compressed dorsoventrally... 66. 



66. Cyrtoceracones XLVII . Tripteroceras. 



66. Gyroceracones XLVIII. Edaphoceras. 



**. Close-coiled gyroceracones, the whorls in contact, or nautilicones with 

 more or less pronounced impressed zone d. 



d. Surface without annulations or nodes 77. 



77. Whorls not angulate ; venter flat or rounded cc. 



cc. Whorls in contact only or very slightly depressed, the last often 



free ftf. 



■ftf. Venter narrower than dorsum b. 



b. Siphuncle near venter XVII, Aphetoceras. 



b. Siphuncle near center XVIII. Barrandeoceras. 



■f-ff. Section nearly or quite circular (more rarely subquad- 

 rangular) c. 



