128 PAL.EONTOLOGY OF NEW-VORK. 



Figs. 2i,k,l,m, Front views, showing the increasing sinus of the dorsal valve as the shell becomes 

 older. 



Plate XXXII. B. 



Figs. I a, 4. Cardinal views of a young and old specimen of Orthis occidentalU. 



Fig. 1 e. Ventral valve of an imperfect specimen, partially showing the interior of the dorsal valve, 



which is strongly marked by the external radii. 

 Fig. 1 d. Profile view of a large specimen. 

 Figs. I e,f,g. Views of ventral valves. 

 Figs. 1 A, *. Interior of ventral valves of the same species, showing the medial tooth and the marks of 



the external radii 



Position and locality. This species is associated with the preceding, in considerable 

 numbers, in the Blue limestone of the West ; being found at Maysville ( Ky.), Cincinnati 

 and Oxford (Ohio), Madison (Indiana), and other places. {State Collection.) 



170. 17. ORTHIS SINU ATA (n.*p.). 



Pi. XXXII. B. Figs. 2a-*. 



Semioval, with a sinus in front ; cardinal line scarcely equal to the width of the shell ; 

 dorsal area large, triangular ; foramen triangular, reaching to the beak, the upper margins 

 sloping rather abruptly from the beak ; ventral area narrow linear, foramen broad triangu- 

 lar, with a distinct medial tooth, which reaches as high as the area ; dorsal valve convex, 

 its greatest elevation at the point of the beak, which is acute ; a depression along the centre, 

 which becomes a sinus in older shells ; ventral valve regularly convex in young specimens, 

 gibbous, somewhat emarginate and elevated in front in older specimens ; surface marked 

 by strong, regular, rounded striae, which bifurcate in a nearly uniform manner about half 

 way to the base ; striae crossed by elevated sub -imbricating concentric lines. A few 

 imbricating lines of growth are distinct towards the margin of the older shells. 



This species is distinguished from the last by the stronger and more prominent striae, 

 which are likewise more regularly bifurcating. The beak of the dorsal valve is more 

 elevated and acute, giving a greater height to the dorsal area. The ventral valve is about 

 equally convex or gibbous with the last, while it never exhibits any depression along the 

 centre. The depression or sinus in the dorsal valve is usually more abrupt, deeper, and often 

 accompanied by a corresponding elevation on the ventral valve, which docs not occur in 

 the preceding species. The young shells of the species under consideration are more gibbous 

 than the last, and have the beak of the dorseil valve more elevated and acute, differing 

 conspicuously in this respect from the previous one. 



The variations produced by age, and the difficulty of obtaining a series of specimens, 

 often produce confusion in regard to these similar species, and it will frequently be found 

 a difficult task to make the proper disposition of specimens. 



