948 PALAEONTOLOGY OF NEW- YORK. 



Spirifera alta ( n. s.)- 



PLATE XLIIL 

 Shell semipyramidal, triangular in form from the ventral or cardinal sides, 

 and semioval in a dorsal view ; cardinal line equalling the greatest 

 width of the shell, and sometimes produced in short auriculate exten- 

 sions : surface plicate. 

 Venteal valve extremely elevated, subtriangular ; height and width 

 about as three to five : sinus broad rounded in the bottom, and ex- 

 tended from beak to front. Area very large, triangular, acute, inclining 

 forward or nearly rectangular to the general plane of the dorsal valve, 

 strongly striated vertically. The fissure is large but comparatively 

 narrow, being more than twice as high as its width at the base, and is 

 closed by a concave transverse septum for two-thirds of its length from 

 the apex. The margins of the fissure are grooved, as in other species, 

 for the reception of a pseudo-deltidium. 

 Dorsal valve large, somewhat semioval, depressed-convex or but mode- 

 rately convex at the sides, with a broad and well-defined mesial fold : 

 the cardinal angles are sometimes obtusely rounded or truncate, and 

 sometimes a little produced. Area comparatively wide, and strongly 

 striated vertically. 

 SoKFACB of the dorsal valve plicated by about twenty-five to thirty or 

 more low rounded ribs on each side of the mesial fold : in the casts, 

 half the number are often quite obscure towards the extremities. The 

 mesial fold and sinus are plicated, and the ribs are faintly preserved, 

 or sometimes barely visible in the casts. In casts of the ventral valve, 

 the plications are scarcely preserved except towards the margin. 

 There is a distinctly ovate muscular impression in the ventral valve, 

 •with indications of a low crest in the centre ; while the dental lamellae 

 we only extended for half the length of the muscular space, yet they 

 continue to form the walls of the fissure to its base ; but we have no 

 evidence of the teeth, except in the character of the teeth-sockets in 



