184 
PALAEONTOLOGY OF NEW-YORK. 
sloping sides of which, and the intermediate spaces, are marked by beautifully 
undulating stria; having a different character from those usually seen in the Ten- 
taculite limestone, but resembling tliose of fig. 1, Plate xvi, which is a very flat 
shell. The cast still preserves the marks of the stride, which are punctate; and the 
interior shell is distinctly papillose along the lines of striae. 
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 the discrimination of species, unless upon a comparison 
of a large number of specimens. 
Fig. 1 a, b, c, d. Ventral valves of several specimens where the shell is more or less ex¬ 
foliated, the last one being almost free from adhering shell. 
Fig. 1 e,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 : Becraft’s mountain, Hudson. 
Strophodonta planulata (n. s.). 
Plate XVI. Fig. 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. 
a 
Surface 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. 
