250 NORTH AMERICAN INDEX FOSSILS. 



\ f . Strong mesial fold on brachial valve and corresponding sinus on 



pedicle valve LIII. Plaiystrophia. 



\ f . No mesial fold present. Shallow sinus present or absent. 



XLIX. Plectorthis, 



B. Shell surface radially striate (i. e., radiating lines fine) **. 



** Strongly two-lobed. Very small L1V. Bilobites. 



** Not two-lobed 2. 



2. Hinge line short (forming about one third the greatest width of shell)., b. 

 b. Valves subequally and gently biconvex LVI. Rhipidomella. 



b. Brachial valve very convex ; pedicle valve much less convex. 



LVII. Schizophoria. 

 2. Hinge line long c. 



c. Shell substance punctate LV. Dalmanella. 



c. Shell substance impunctate i f . 



2 / . Muscular impression of pedicle valve small, almost confined be- 

 tween the dental lamella L1I. Orthostrophia. 



2'. Muscular impression of pedicle valve moderately large, extend- 

 ing about half the distance from the beak to the front of the 

 shell LI. Hebertella. 



XLVIII. Orthis Dalman (emend Hall and Clarke). 

 Brachial valve flat; pedicle valve convex. Cardinal area of 

 pedicle valve elevated and somewhat incurved. Surface covered 

 with strong, sharp, and comparatively few plications which are 

 usually if not always simple. Dental lamellae slightly developed. 

 Cardinal process, a vertical plate lying at the bottom of the brachial 

 deltidial cavity, and longitudinally dividing it. Camb.-Carb. 



162. 0. costalis Hall. (Fig. 300, a-b.) Ordovicic. 



Hinge line slightly less than the greatest width of the shell. 



Pedicle valve very convex with high cardinal area and beak not 





<5 a 



Fig. 300. a, 6, Orthis costalis ; c, d, Plectorthis fissicosta. (After Hall.) 



incurved. Surface covered by about 30 distant, strong and rounded 

 plications. 



Chazy of New York. 

 163. 0. tricenaria Conrad. (Fig. 301, a-c.) Ordovicic. 



Distinguished from 0. flabellites by its smaller size and strongly 

 convex pedicle valve with very high cardinal area. 



Trenton ; widely distributed throughout North America. 



