TKENTON LIMESTONE. 119 



incurved ; surface marked by fine, close, radiating striae which are unequally bifurcated 

 towards the margin, and transversely marked by prominent imbricating squamose lamellae, 

 which alternately advance and retreat in passing over the striae and the spaces between 

 them. Width half an inch ; length less than half an inch. 



This species is, in form, much like 0. subaquata ; but the valves are less convex, and it 

 is readily distinguished by the squamose striae, which give the surface a rugose appearance, 

 differing from all the other species of Orthis in this rock. 



Fig. 3 a. Ventral valve. 



Fig. 3 b. Dorsal valve. 



Fig. 3 e. Profile view, showing the nearly straight projecting beak of the dorsal valve. 



Fig. 3 d. Cardinal view, showing the large triangular foramen. 



Fig. 3 e. An enlarged portion of the suriace, showing the imbricating squamose lamellae. 



Position and locality. This unique and beautiful species occurs with the preceding, and 

 others which are common to the Trenton limestone of New- York, though it has not yet 

 been noticed within this State. 



158. 5. ORTHIS DISPARILIS. 



Pl. XXXII. Figs. 4 a, b, c. 

 Orthit duparilis. Conkad, Ptoc. Acad. Nat. Sciences, 1S43, Vol. i. p. 333. 



Semicircular, with about 28 regular rounded striae, more than half of which reach the 

 beak, the intermediate ones commencing on the umbo ; striae crossed by finer concentric 

 elevated lines ; ventral valve flat or slightly concave, depressed along the centre, which 

 produces a slight curvature in the edge of the shell ; margin of the shell regularly rounded 

 from the extremities of the cardinal line ; area large ; foramen a narrow nearly linear slit, 

 reaching to the apex. 



This peculiar shell is readily identified by iu greatly elevated or subcorneal dorsal valve, 

 and quite flat or slightly concave ventral valve. The striae are stronger than in the preceding 

 species, and uniformly crossed by concentric lines as in 0. testudinaria. The dorsal area 

 is proportionally larger than in any other species except 0. tricenaria, which is a larger 

 and more elongated shell. 



Fig. 4 *. Ventral valve. 



Fig. 4 b. Dorsal valve. 



Fig. 4 e. Profile view, showing the great disproportion in the size of the valves. 



Fig. 4 d. View of the cardinal area and foramen. 



Position and locality. This species occurs at Mineral Point ( Wisconsin), associated with 

 the preceding species. I have also seen it from Cincinnati, associated with numerous species 

 of Brachiopoda typical of the Trenton limestone. 



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