81 

 Orthis Hippolytb. (N. sp.) 



Fig. 73. a, b, c, d, e,f. 



Description. — Sub-quadrate, hinge-line equal to the greatest width of the 

 shell ; cardinal angles rectangular, or a little obtuse, sometimes shghtly 

 rounded ; sides in the upper one-third, straight, or gently convex ; front 

 angles and margin broadly rounded. Ventral Talve strongly convex in 

 the small, and moderately so in large specimens ; more or less carinated 

 along the middle, -with a Eat slope towards the sides ; greatest elevation 

 about the middle or a little above ; beak depressed to about half the height 

 of the shell ; area of moderate size, forming an angle of about 125° with 

 the plane of the lateral margin ; foramen rather small. Dorsal valve 

 gently convex in the upper half, with a broad, shallow mesial sinus 

 extending from the front margin to the umbo, a flat slope to the cardinal 

 angles ; area almost linear ; beak scarcely distiuct from the cardinal edge. 



Surface with moderately strong, , angular, radiating, bifurcating ribs, 

 three or four in the width of one hne at the front margin, becoming finer 

 towards the cardinal angles ; usually, one, two, or three small ones in the 

 spaces between the larger, which do not reach the beak. 



Width, from 3 to 6 luies; length from one-fifth to one-third less than the 

 width. 



This species is alhed to 0. orthamhonites, but it has the ribs always 

 sub-divided, and it varies greatly in size. It is also more distinctly 

 carinated along the middle. 



Locality and Formation. — Limestone No. 2, at Point L^vis. Also in 

 the upper part of the limestone at Phillipsburgh. 



Collectors. — Sir W. E. Logan, J. Richardson. 



Oethis Evadnb. (N. sp.) 



Fig. 74. a, b, c, d. 



Description. — Subquadrate ; hinge-line straight, equal to the greatest 

 width of the shell ; sides in the upper two thirds nearly straight, or gently 

 convex ; front angles rounded ; front margin rounded, or with a portion in 

 the middle equal to half the whole width nearly straight. Ventral valve 

 moderately convex, greatest elevation a little in front of the middle, the 

 upper half somewhat flat and sloping to the hinge-lme, the front half the 

 most convex, the surface descendmg from the middle to the front margin 

 and sides with an abruptly rounded slope ; umbo very slightly elevated 



