46 [Senate 



Orthis steophomenoides ( n. s.). 



Pal. N.Y. Vol. iii, pi. 14, f. 2. 



Shell transverse, somewhat semioval : ventral valve flattened 

 convex, with a distinct narrow mesial elevation passing from 

 beak to base ; beak scarcely distinct from hinge-line, straight : 

 dorsal valve more convex than the opposite, most elevated be- 

 tween the middle and the beak, from which a distinct narrow 

 depression extends towards the front ; beak more prominent 

 than the opposite, obtuse, incurved : hinge-line straight, nearly 

 equalling the greatest width of the shell ; area large, plane ; 

 foramen apparently closed. Surface marked by coarse radiating 

 striae, which increase by implantation and bifurcation : several 

 of those on the mesial elevation of the ventral valve appear to 

 coalesce along the centre, before reaching the beak. In well 

 preserved specimens, strong concentric strise are visible in the 

 depressions between the radiating strise. Shell marked by a few 

 strong concentric undulations of growth. 

 Kesembles very nearly O.fasciata of the Niagara group. 



Orthis musculosa ( n. s.). 



Pai: N.Y. Vol. iii, pi. 91, f . 1 - 3. 



Shell suborbicular, the length about nine-tenths as great as the 

 width : ventral valve depressed-convex, sometimes slightly 

 concave near the front ; beak prominent, equalling or extending 

 a little beyond that of the opposite valve, pointed and slightly 

 incurved : dorsal valve regularly and distinctly convex, most 

 elevated in the central region, sometimes a little depressed to- 

 wards the front ; beak prominent, triangular, pointed and in- 

 curved ; cardinal teeth and process strong ; hinge extremely 

 short ; area triangular, scarcely extending beyond the foramen ; 

 foramen large, partly occupied by the prominent cardinal pro- 

 cess of the other valve , visceral impression large, fan-shaped, 

 and strong. Surface marked by fine, distinct, radiating strise, 

 those nearest the cardinal margin being curved outwards from 

 the beak J concentrically marked by obscure lines of growth. 



