184 PALEONTOLOGY OF NEW-YORK. 



sloping sides of which, and the intermediate spaces, are marked by beautifully 

 undulating strise having a different character from those usually seen in the Ten- 

 taculite limestone, but resembling those of fig. 1, Plate xvi, which is a very flat 

 shell. The cast still preserves the marks of the striae, which arc punctate; and the 

 interior shell is distinctly papillose along the lines of strlse. 



The rock in which this form occurs is a brecciated semicrystalline limestone, 

 which has retained the shells in a good degree of preservation; but the exterior 

 surface usually adheres to the stone, leaving casts of the fossils. 



The variations in form, convexity, and surface markings of this species, if we 

 include all at present thus indicated, show very conclusively that such characters 

 are not to be relied on in thg discrimination of species, unless upon a comparison 

 of a large number of specimens. 



Fig. 1 a, b, c, d. Ventral valves of several specimens wliere the shell is more or leas ex- 

 foliated, the last one being almost free from adhering shell. 



Fig. 1 «, f, g. Profile views, showing the convexity of several individuals. 



Fig. 1 h. The surface showing the ridges and striae. 



Fig. 1 I. A portion of a cast of the interior, which preserves the ridges, but shows no in- 

 termediate striae. 



Geological position and locality. In a crystalline band of the shaly limestone of 

 the Lower Helderberg group : Becruft's mountain, Hudson. 



Strophodonta plannlata ( n. s.). , 



Plate XVI. Fio. 9-12. 



Shell semielliptical, width nearly one-half the length, plano-convex : 

 hinge line greater than the width of the shell below; the cardinal 

 extremities often salient. Dorsal valve flat. Ventral valve uniformly 

 and very slightly convex, sometimes flattened towards the margins : 

 beak scarcely elevated above the hinge line. Area linear. Foramen 

 unknown. 



SoKFACE finely and evenly striated : striae of the dorsal valve often flat- 

 tened. Radiating striae crossed by fine closely arranged concentric 

 striae, and sometimes with a few inconspicuous laminae of growth, and 

 towards the cardinal extremities by a few wrinkles or undulations. 



« • 



