ORTHIDES OF THE UPPER HELDERBERG ROCKS. 35 



The casta of these two valves, which I have placed under one designation, 

 have not been found in actual connexion ; but the character of the ventral valve 

 requires a dorsal valve of the form described, and vice versa; wliile the two oc- 

 curring in the same locality, and of equal rarity, I can feel little hesitation in 

 considering them of the same species. 



Geological formation and locality. In the Schoharie grit : at Clarksville and 

 Knox, Albany county, N.Y. 



Ortliis lenticularis. 



PLATE V. 



Orlhis Uriticularit [1 ] Vakuxem, Geological Report Third District p. 133, f. 4. 

 Compare Orthu tolitaria of the Hamilton group. 



Shell suborbicular or subquadrate, sometimes suboval, greatest width 

 above or below the middle, subplanoconvex : hinge-line equalling or 

 greater than two-thirds the greatest width of the shell ; cardinal extre- 

 mities rounded. Dorsal valve varying from very depressed-convex to 

 moderately convex in the upper part, more or less abruptly depressed 

 towards the cardinal extremities, which are a little deflected ; gently 

 curving towards the middle of the sides, and the centre marked by a sinus 

 which begins at the beak and expands so as to produce a broad depres- 

 sion in front : area linear, the beak projecting a little beyond the area 

 line. Ventral valve very convex, sometimes obtusely subcarinate along 

 the centre ; greatest convexity near, or a little above the middle of the 

 valve, curving very abruptly to the front and to the sides (or, when 

 carinate, it is irregularly depressed ), and curving more gently towards 

 the beak : area of moderate width, a little incurved ; foramen com- 

 paratively large, wider than high ; beak incurved over the foramen. 



Surface finely striated with rounded subequal radiating strias, which bi- 

 furcate at intervals, and sometimes every fifth, sixth, or seventh one 

 is more prominent than the others : about sixteen to twenty-two stria3 

 in the space of two lines, the stria3 wider than the spaces between them, 

 and crossed at unequal distances by subimbricating lines of growth. 

 Entire surface finely papillose, and, on the worn surfaces, punctate. 



