ORTHIDES OF THE UPPER HELDERBERG ROCKS. 
35 
The casts 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; while 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. 
Orthis lenticularis. 
PLATE Y. 
Orthis lenticularis [? ] Vanuxem, Geological Report Third District p. 139, f. 4. 
Compare Orthis solitaria 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 striae, which bi¬ 
furcate at intervals, and sometimes every fifth, sixth, or seventh one 
is more prominent than the others : about sixteen to twenty-two striae 
in the space of two lines, the striae 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. 
