IT« PALJCONTOLOGY OF NEW-YORK. 



The cardinal process is small in young shells, thickened in older ones, and does 

 not rise as high as the brachial processes. In the latter character this species contrasts 

 with 0. subcarinata, where the cardinal process rises above the brachial lamellae. 

 In species so nearly allied as the two here indicated, every feature of the shell, 

 and of its internal structure, requires careful comparison; and it may often occur 

 that in a single character the two approach so nearly as to furnish no reliable means 

 of separation, while the sum of the characters renders the distinction conspicuous. 



Fig. 4 a, b, c, etc. Views of small individuals, which appear to be the young of this species, 



but may prove distinct. 

 Fig. 5 a, b, c. Small individuals, authentic forms of this species. 

 Fig. 6 a, b, c. Specimens intermediate in size. 

 Pig. 7 a, b. Specimens of the ordinary size. 

 Fig. 8. A large individual of this species. 

 Fig. 9 a, 6. Interior of the ventral valve. > 

 Fig. 10 a, b, e. Interior of the dorsal valve. 



Fig. 10 d. Profile showing the comparative elevation of the cardinal and brachial processes. 

 Fig. 11 a, b. Casts of the ventral valve. 

 Fig. 12 a, b. Casts of the dorsal valve. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Helderberg mountains; Catskill, Hudson, Schoharie, etc. 



Orthis concinna ( n. s.). 



Plate XIII. Fig. 1-3. 

 Compare Orthis parva. Pander, Vem. M. V. K. Geol. Russ. pa. 188, pi. 13, f. 3. 



Shell longitudinally semielliptical : valves unequally convex : hinge line 

 straight, with the extremities subangular. Dorsal valve convex, with a 

 depression from beak to base, on each side of which the shell is more 

 convex, and thence sloping somewhat abruptly to the sides : beak 

 very small, and scarcely incurved. Ventral valve very convex, gibbous, 

 and almost subcarinate in the middle : beak prominent, much elevated 

 above the hinge line, and neatly incurved over the area. Area com- 

 paratively large, the length greater than half the width of the shell. 



ScBFACE very finely and evenly striated. 



