1 86 NORTH AMERICAN INDEX FOSSILS. 



20* No deltidium present 18". 



18" Shell cyrtiniform with erect cardinal area. 



LX. Scenidium. 



18" Shell not cyrtiniform XLVIII. Orthis. 



19/j- Brachial valve convex 21* 



21* Strong mesial fold on brachial valve and corre- 

 sponding sinus on pedicle valve, both with 



sh arp pli cation s LI 1 1 . Platystrophia. 



21* No mesial fold present. Shallow sinus present or 



absent XLIX. Plectorthis. 



I. Surface radially striate (*. <?., radiating lines fine; except Hebertella 



borealis 1 74 ) IIII 



mi. Strongly two-lobed. Very small LIV. Bilobites. 



IIII. Not strongly two-lobed ww. 



ww. Hinge line short (about y^ the greatest width of shell). 2of. 

 20f Valves subequal and gently biconvex. 



LVI. Rhipidomella. 

 2of Brachial valve very convex. Pedicle valve much less 



convex LVII. Schizophoria. 



ww. Hinge line long, pedicle valve deep, brachial valve flat or 



shallow 21 f 



2 If Shell substance punctate LV. Dalmanella. 



2if Shell substance impunctate 22* 



22* Pedicle valve subpyramidal. Deltidium and 



spondylium present LIX. Clitambonites. 



22* Pedicle valve depressed convex 19". 



ic/' Muscular impression of pedicle valve small, 

 almost confined between the dental la- 

 mellae LII. Orthostrophia. 



19/' Muscular impression of pedicle valve mod- 

 erately large, extending about one half 

 the distance from beak to front. 



LI. Hebertella. 

 4. Shell spiriferoid — Usually transversely elongate with hinge line forming 

 greatest width of shell. Bi-convex, very seldom plano-convex. 

 Cardinal area with the open triangular delthyrium most conspicuous 

 on the pedicle valve, rarely with pseudodeltidium. Median sinus on 

 pedicle valve and fold on brachial (reversed in Metaplasia, Vitulina, 

 Scenidium'). Shell usually radially plicate, rarely smooth or striate. 



Brachidium typically an extraverted spire m. 



m. Conspicuous spines present. 



CIV. Reticularia (except R. nevadaensis, R. Iccvis and R. perplexa). 



m. Minute spines present 324. Spiriferina spinosa. 



m. Spines absent or microscopic 2222. 



2222. Surface lacking plications xx. 



xx. Mesial fold and sinus normal in position (i. e., fold on 



brachial valve and sinus on pedicle) 22f. 



22f Surface smooth. 



CVII. Amboccelia (except A. plan oconvexa)^ 

 22f Surface with concentric striae 23*. 





