137 



those figured in the works above cited, but are in every other respect spe- 

 cifically identical. The area varies much in its size and in the amount of 

 its inclination. 



Locality and Formation. City of Ottawa ; Belleville ; and near 

 L'Orignal. It also occurs at Cincinnati in Ohio, in the Hudson River 

 group, and at numerous localities in Wales, in the Bala group. 



Collectors. Sir W. E. Logan, R. Bell, E. Billings. 



ORTHIS MARIA. (N. sp.) 



Fig. 114. 

 Fig. 114. Orthis Maria, a, ventral view ; 6, side view; c, view of the front margin. 



Description. Transversely oblong ; hinge-line equal to or a little less 

 than the greatest width ; cardinal angles either rectangular or moderately 

 rounded ; sides gently convex and subparallel ; anterior angles rounded ; 

 front margin with a portion of the middle usually straight. Ventral 

 valve irregularly depressed pyramidal, with a deep rounded mesial sinus 

 which becomes obsolete before reaching the beak; the latter small, 

 pointed, not incurved, scarcely distinct from the cardinal edge, and form- 

 ing the most elevated point of the valve. From the beak to the cardinal 

 angles the shell descends with a flat or sometimes gently concave slope. 

 Area rather large, its height at the beak equal to one eighth of its 

 length, slightly concave, nearly at a right angle to the plane of the mar- 

 gin, but slightly overhanging the hinge-line. Dorsal valve rather strongly 

 convex, with its front much elevated in the middle by a large rounded 

 mesial fold which becomes obsolete at the umbo ; cardinal angles com- 

 pressed and slightly reflected ; umbo large, obtusely rounded, overhang- 

 ing the hinge-line ; beak very small but distinct, situated on the cardinal 

 edge and not elevated ; area about one third the size of that of the ventral 

 valve, lying nearly in the plane of the margin, slightly concave and some- 

 times a little overhanging the hinge-line. Foramen of ventral valve tri- 

 angular, its width at base about two thirds the height, apparently open to 

 the beak, but this point not clearly ascertained. Surface with rather fine 

 radiating striae, 3 or 4 in the width of a line at the middle of the front 



