270 NORTH AMERICAN INDEX FOSSILS. 



crura. Entire surface of shell, including the few more or less 

 strong plications, covered with fine radiating striae. Upper Car- 

 bonic. 



214. E. hemiplicata Hall. (Fig. 324, a-d.) Upper Carbonic. 

 Surface covered with fine and crowded radiating striae and a few 



large and subangular radial plications which are distinct anteriorly 

 and die out towards the umbo. They are crossed near the front 

 by a few concentric lines of growth. 



Missouri, Illinois, Iowa, Nebraska, Colorado. 



LIX. Clitambonites Pander. 

 Semicircular. Hinge line straight, forming greatest diameter of 

 shell. Pedicle valve convex or subpyramidal ; cardinal area high 

 and usually vertical with a broad convex deltidium perforate at 

 apex ; on the interior the dental plates unite to form a spondylium 

 which is supported on a median septum. Brachial valve depressed 

 convex or flattened. Upper Cambric-Ordovicic. 



215. C. diversus (Shaler). (Fig. 324, e-g.) Ordovicic. 

 Subquadrangular. Umbo of pedicle valve always laterally in- 

 clined toward either extremity of the hinge line. Surface marked 

 by numerous prominent striae crossed by delicate crowded growth 

 lines. 



Trenton-Lorraine of Wisconsin, Minnesota, Ontario, Manitoba, 

 Anticosti. 



LX. Scenidium Hall. 



Small. Hinge line straight, usually equalling the greatest 

 width of the shell ; delthyrium in each valve. Pedicle valve sub- 

 pyramidal with a high and usually erect cardinal area. Brachial 

 valve depressed, convex to concave, with low cardinal area. The 

 small cardinal process extends forward as a median septum through 

 the length of the valve and is at times so developed anteriorly as to 

 reach into the pedicle valve. Ordovicic-Devonic. 



216. S. anthonense Sardeson. (Fig. 324, h-k.) Ordovicic. 

 Small with extended hinge line and conspicuous mesial sinus 



and fold. Surface radiately striated. 



Trenton of Tennessee, Illinois, Minnesota. 



LXI. Syntrophia Hall and Clarke. 



Transversely elongate, biconvex. Hinge line nearly equalling 

 the greatest width of shell. Cardinal area long and straight; an 

 open delthyrium on each valve. Spondylium deep and short, its 

 anterior half unsupported. Upper Cambric-Ordovicic. 



