314 NORTH AMERICAN INDEX FOSSILS. 



from C. umbonata in its smaller size, wider hinge line and higher 

 cardinal area. 



Chouteau of Missouri. 



CI. Cyrtia Dalman. 



Semipyramidal shells with cardinal area vertical, that of the 

 pedicle valve high and triangular. Delthyrium narrow, generally 

 open. Otherwise like Spirrfer. Siluric-Mississippic. 



323. C. alta Hall. (Fig. 394.) Mississippi. 



Semipyramidal with hinge line equaling the greatest width of 



shell. Pedicle valve very elevated ; height to width about as three 



to five ; sinus broad and rounded^ 

 cardinal area vertically striated. Bra- 

 chial valve depressed-convex; car- 

 dinal area comparatively wide ; sur- 

 face marked with 25 to 30 low, 

 rounded plications on each side of 

 the median fold. Median fold and 

 sinus faintly plicated. 



Waverly of Pennsylvania and 

 Ohio. 



CII. Spiriferina d'Orbigny. 



Differs from Cyrtina in that the 

 dental plates do not unite with the 

 high median septum and in that the 

 valves are less unequal in size. 

 Shell substance punctate. Devonic ? Mississippic-Jurassic. 



(Fig. 395-) 

 Mississippic-Carbonic. 



Of less than medium size, semioval. Brachial valve with prom- 

 inent median fold. Pedicle valve slightly more convex than the 

 opposite with strongly defined mesial 

 sinus and slightly arched beak Sur- 

 face bearing four or five sub-angular or 

 rounded plications on each side the . ., . 



. 1 1 • 1 • Fig. 395. Spiriferina spinosa, 



fold and Sinus, crossed by imbncat- a Mississippi Valley and a Nev- 



ing lamellae of growth. Whole surface ada specimen. (After Walcott.) 



Fig. 394. Cyrtia alta X 

 N. Y. VIII.). 



(Pal. 



324. S. Spinosa (Norwood and Pratten). 



