122 PALEONTOLOGY OF NEW-YORK. 



Fig. 8 a. View of the ventral valve and area ; the specimen imperfect. 



Fig. 8 b. laterior of the dorsal ^alve, showing the cardinal area. 



Fig. 8 c. Exterior of the dorsal valve. 



Fig. 8 d. Interior of the ventral valve, shovying the toothlike processes and visceral impression. 



Fig. 8 e. Profile view of a perfect specimen. 



This is a rare sliell in New-York, and I have been able to obtain but few specimens, and 

 these for the most part imperfect. Its associates in the strata are Orthis testudinaria, Ltptana 

 sericea, ^trypa triplex, Ceraurus pleurexanthemus, ^c. 



Position and locality. In New- York, it occurs in the lower part of the Trenton limestone, 

 associated as above, at Middlcville. It is found at Mineral Point (Wisconsin), apparently 

 in the higher part of the same rock. {t<tutc Collection.) 



163. 10. ORTHIS PLICATELLA (71.5^;.). 



Pl. XXXII. Figs. 9 a, b, c, <l, e,f, g. 

 Compare Orthis radians. J. Sowerby in Sil. System, pag. 639, pl. 23, f. U. 



Broadly semioval, nearly equivalve, length and breadth aJjout as .3 to 4 ; sttrface marked 

 b}' strong radiating plica-, which are usually simple, about 20 to 28 on each valve, crossed 

 by simple elevated concentric lines, which are more distinct in the depressions between the 

 costsE, and often obscure or obsolete upon, their exposed surfaces j valves nearly equally 

 convex, without sensible depression or elevation on either one, meeting at the edges in a 

 straight line ; cardinal line not extending beyond the width of the shell ; area narrow ; 

 dorsal foramen extending to the beak. 



This species bears some resemblance to Orthis radians of Sowerby, but it proves clearly 

 distinct on comparison with his description ; the plications are never so few as 15, and the 

 shell is never concave in front. The equal convexity of the valves, and uniform, strong, 

 somewhat sharp plications, are distinguishing characters. The proportionate greater exten- 

 sion of the hinge line, and less distance from beak to base, distinguish tliis shell from any 

 variety of 0. pedinella, even when the surface only can be seen. 



Fig. 9 a. Ventral valve of midium size. 



Fig. 9 b. Ventral valve of a small individual. 



F)g. 9 c. Ventral valve, showing the apex and part of the dorsal area. 



Fig. 9 d. Profile view of the same. 



Fig. 9 e. Cardinal view, showing the dimensions of the area and foramen. 



Fig. 9/. Ventral valve of a large specimen ; the plications trifid towards the base. 



Fig. 9 g. ProfiJe view of the same. 



Position and locality. A single well marked specimen only, has fallen under my observa- 

 tion within the Slate (at Middlevillc), where it occurs in the lower part of the Trenton 

 limestone. It is quite frequent at Cincinnati (Ohio), Maysville (Ky.), Madison (Indiana), 

 associated with the Leptccna alternata, L. sericea, Orthis testudinaria, and other fossils 

 typical of the Trenton limestone. (State Collection.) 



