LOWER HELUERBERG ROCKS. 183 



Fig. 6. An individual with coarser strise than usual, the surface partially exfoliated. 



Fig. 6 a. The striae enlarged, showing the punctate character of the surface. 



Fig. 7 a, h. Illustrations of the surface of Leptana indenta. 



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 6. 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 altemata 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 unfrequently associated 

 with Strophodonta mglypha, Rhynchonella semiplicata and Pentamerus galeattis. 



Strophodoiita vaiistriata, vkr. arata. 



Plate XVIII. Fig. 1 a - t". 



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 shaly 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 difierent 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 



