256 SYSTEMATIC PALEONTOLOGY 



Genus CHRISTIANIA Hall and Clarke 

 CHRISTIANIA TRENTONEXSIS Euedemann 



Plate XLVIII, Figs. 16-18 



Christiania trentonensis Ruedemann, 1902, Bull. New York State Mus., 

 No. 49, p. 21, pi. ii, figs. 2-6. 



Description. " Shell small, convexo-concave, somewhat variable in 

 shape, rotundo-quadrate to rotundo-rectangular ; sides sub-parallel or 

 slightly converging to the cardinal line; front rounded. Hinge line 

 straight, only slightly shorter than the greatest width of the valve in the 

 middle part; cardinal extremities obtusely angular, having the appear- 

 ance of flattened ears. Pedicle valve uniformly and strongly convex; 

 umbo slightly projecting and very narrow ; beak obscure. Cardinal area 

 narrow ( ?) ; interior of pedicle valve not observed. Brachial valve 

 strongly concave, beak hardly projecting beyond the long, straight hinge 

 line. Cardinal extremities strongly developed, flat; area very small, 

 cardinal process small, bipartite on its anterior face; the lobes being 

 denticulate anteriorly with from three to five small denticles on each 

 side. Crural plates very long and slightly divergent; the lower portion 

 produced on each side as a strongly elevated wall with perpendicular 

 sides extending in the original direction of the crural plates close to the 

 ante-lateral angle, where it recurves and returns, parallel to the median 

 axis and nearly in a straight line as a still more prominent wall merging 

 into the base of the cardinal process. The elongate, symmetric, sub- 

 rectangular spaces thus formed are each divided transversely by a vertical 

 ridge about one-third of the length of the valve from the cardinal line. 

 The long narrow space between the inner muscular walls is also bounded 

 anteriorly by a low, rounded, curving ridge and divided in the median line 

 of the shell by a low, rounded, longitudinal ridge. The anterior half of 

 the surface of the long anterior adductors is very rugose and radially 

 striated. 



" The surface is covered with concentric lines of growth and radiating 

 quite widely separated, filiform striae with smooth, flat, interspaces." 

 Ruedemann, 1902. 



