NIAGARA GROUP. 



255 



rarely appearing on the elevations ; a few strong imbricating lines of growth near the base ; 

 cardinal area usually narrow, and extending to the extremity of the hinge-line. 



This shell is usually much compressed, and both valves are flattened so as to appear equal. 

 In more perfect specimens, the dorsal valve is more convex than the ventral. The foramen is 

 broadly triangular, with a thin sharp tooth in the centre, and a stronger one on each side pro- 

 jecting into the centre ; the muscular impression has a strong rounded ridge down the centre, 

 with a depression on each side, but the margins are not well defined. The interplications, on 

 the inside, appear to be duplicate, or have a groove along the centre. In some specimens, tha 

 plications on the interior extend but halfway to the beak ; while in others, that are apparently 

 of the same species, they extend to the muscular impression. 



Fig. 6 a, b, c. Dorsal, profile and ventral view of a compressed specimen. 



Fig. 6 d. The interior of a ventral valve, showing the characters described above. 



Fig. 6 e. A cast of a ventral valve, having similar characters with the preceding, except that 



the marks of plications extend nearly to the beak. 

 Fig. 6 /. Cardinal view of a specimen, which is partially a cast, having external characters similar 



to fig. 6 e. 

 Fig. 6 g. Cardinal view of the specimen fig. 6 a. 



Position and locality. In the shale of this group at Lockport, Rochester, Sweden, Wolcott 

 and other places. 



616. 29. ORTHIS FLABELLULUM? Var.l 



PL. LII. Fig. 7 a-d. 



Specimen robust, both valves nearly equally convex ; plications simple, strong, rounded, 

 crossed by concentric strice, and near the base by stronger lines of growth ; area large, partly 

 occupied by the ventral valve ; beak of the dorsal valve much elevated. 



I can find no essential characters in the single specimen I possess, to separate it from the 

 preceding, if the cast fig. 6 e and the specimen fig. 6 _/ are admitted under this species. For 

 the present, therefore, I shall unite the two. 



Fig. 7 a, b. Dorsal and ventral views of the specimen. 

 Fig. 7 c. Profile view of the same. 

 Fig. 7 d. Cardinal view of the same. 



Position and locality. In the shale of the Niagara group at Rochester. 

 617. 30. ORTHIS FASCIATA (71. ^y).). 



PL. LII. Fig. 8 a, b. 



Shell semioval ; hinge-line sometimes extended beyond the width of the shell ; surface 

 striated ; striee in fascicles, which are almost simple at their origin, and divide into two prin- 



