81 



ORTHIS HIPPOLYTE. (N. sp.) 



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



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

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

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

 angles and margin broadly rounded. Ventral valve strongly convex in 

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

 along the middle, with a flat 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 distinct from the cardinal edge. 



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

 three or four in the width of one line 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 lines ; length from one-fifth to one-third less than the 

 width. 



This species is allied to 0. orthambonites, 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 Le*vis. Also in 

 the upper part of the limestone at Phillipsburgh. 



Collectors. Sir W. E. Logan, J. Bichardson. 



ORTHIS EVADNE. (N. sp.) 



Fig. 74. a, 6, 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-line, the front half the 

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

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



