LOWER HELDERBERG ROCKS. 
183 
Fig. 6. An individual with coarser striae than usual, the surface partially exfoliated. 
Fig. 6 a. The strife enlarged, showing the punctate character of the surface. 
Fig. 7 a , b. Illustrations of the surface of Leptcerm indent a. 
Fig. 8 a. Illustration of the hinge line, area, vascular impression, etc. of the ventral valve, 
showing the apparent foramen, which is due to fracture, and has not the regular 
triangular form of Strophomena. 
Fig. 8 b. Enlargement of hinge line, teeth, etc. of the dorsal valve. 
Geological position and locality. Extremely abundant in the Tentaculite limestone 
and Pentamerus limestone : Helderberg mountains; Hudson, Catskill, Schoharie, 
Carlisle, Jerusalem hill and Dryhill in Litchfield, Herkimer county, and at inter¬ 
mediate points. 
In the Tentaculite limestone, this species presents great variety in its surface 
characters, size, form, and convexity. It bears much general resemblance to Stro- 
-phomena alternata of the Trenton limestone; and the similarity of the latter rock, 
in its dark- thin beds, makes the resemblance still stronger at first view. It occurs 
abundantly in the base of the Pentamerus limestone, but not unfrecjuently associated 
with Strophodonta englypha, "Rhynchonella semiplicata and Pentamerus galeatus. 
§tropliodonta varistriata, var. arata. 
Plate XYIII. Fig. 1 a - i. 
Shell semielliptical, with the cardinal extremities more or less salient: 
hinge crenulate. Dorsal valve more or less concave. Ventral valve 
varying from moderately convex to very gibbous, and sometimes ge¬ 
niculate towards the front. Area narrow. Foramen none, or a narrow 
elevation or callosity in place of it. 
Surface marked by very prominent sharp angular ridges and intermediate 
fine undulating striae, which cover also the slopes of* the ridges. Some¬ 
times a few short wrinkles mark the spaces between the ridges, along 
the cardinal margin towards the hinge extremities. 
This species was arranged and figured with others of the slialy limestone, in the 
belief of its being a very distinct species; but a careful comparison of a large number 
of specimens from the Tentaculite limestone of different localities, as well as from 
the Pentamerus limestone and shaly limestone, leaves so much doubt of any real 
difference among them, that I prefer, for the present, to refer this one to the same. 
The surface, whether with the shell or as a cast, shows the strong sharp ridges, the 
