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NEW YORK STATE MUSEUM 



valve abrupt while the dorsal valve is but gently convex, its beak 

 being so depressed that it is obscure and the valve has a shouldered 

 appearance on account of the broad ' regular convexity across the 

 posterior part, from which there is a gentle slope anteriorlv. The 

 marginal outline is subcircular. In the more progressed type 

 expressed in the Presque Isle outcrops the large and thickened 

 hinge plate is fully developed and was completely perforated at 

 maturity. Likewise the strong adductor muscle scars separated 

 vertically by a low septal ridge have quite the expression they dis- 

 play in fully developed specimens of R. o v o i d e s and they even 

 show the peculiar divergent vascular markings over the posterior 



Rensselaeria atlantica 



slopes which have been heretofore recorded only in a single 

 example of R. ovoides [Pal. N. Y. v. 8, pt 2, pi. 75, fig. 5]. 



These are structural features important to emphasize for no other 

 species thus far described reproduces these details of that well 

 known Oriskany shell so well. In the ventral valve, also, mature 

 shells bear the expression of R. ovoides in their fully devel- 

 oped dental lamellae and deep pedicle pit. The shells of this 

 species in early stages are transverse or subcircular rather than 

 elongate, the increase of length being an acquisition of later growth. 

 The hinge line is straight and extends for the full diameter of the 

 shell giving the latter a semicircular outline. 



On both ventral and dorsal valves a distinct and prominent 

 cardinal area is present. The straight hinge line extending for 

 nearly the entire width of the valve makes this a conspicuous 

 feature, on the dorsal valve the area maintaining its notable width 

 to the extremity of the cardinal line and then quickly losing it 

 on the hinge angles. In the ventral valve this feature is made 



