240 SYSTEMATIC PALEONTOLOGY 



Description. " Shell usually nearly circular in outline, valves about 

 equally convex. Surface marked by from 30 to 40 coarse striae which 

 increase by implantation and bifurcation. The radial striae are crossed 

 by sharp concentric lamellae, producing the rugose appearance which 

 suggested the specific name. From 10 to 15 of the radial striae are 

 stronger than the others, and between each pair of strong striae is a 

 single weaker one, except in the sinus of the brachial valve, where there 

 are two. The pedicle valve is evenly convex, somewhat flattened toward 

 the front, but without a sinus. Hinge line less than the greatest width. 

 Cardinal area not high, nor much incurved. The bachial valve has a 

 narrow median sinus. 



" The shells vary in outline ; some are wider than long, while in others 

 the width and length are about equal. The sides are rounded, and the 

 front is nearly straight in some specimens and rounded in others." 

 Eaymond, 1911. 



Occurrence. CHAMBERSBURG LIMESTONE ( Caryocystites and Echino 

 spherites beds). Pinesburg and Wilson, Maryland: Fort Loudon and 

 Blue Spring, Pennsylvania. 



The original types of this wide-spread species were obtained in the 

 Black River rocks of Wisconsin. 



Collections. Maryland Geological Survey, U. S. National Museum. 



Genus ORTHIS Dalman 



ORTHIS TRICENARIA Conrad 



Plate XLVII, Figs. 4-6 



Orthis tricenaria Conrad, 1843, Proc. Acad. Nat. Sci. Philadelphia, vol. i, 



p. 333. 



Orthis tricenaria Hall, 1847, Pal. New York, vol. i, p. 121, pi. xxxii, fig. 8. 

 Orthis tricenaria. Hall and Clarke, 1892, Pal. New York, vol. viii, pt. 1, 



pp. 191, 193, 221, 228, pi. v, figs. 9-14. 

 Orthis tricenaria, Weller, 1903, Geol. Surv. New Jersey, Pal., vol. iii, p. 151, 



pi. ix, figs. 18-21. 



Description. " Shell plano-convex, longitudinally semi-elliptical in 

 outline ; hinge-line equal to the greatest width of the shell, rarely shorter. 

 Cardinal area well developed on each valve. Surface marked by 30 to 3fi 



