202 



NEW YORK STATE MUSEUM 



curved, usually exposing the circular apical foramen, beneath which 

 the deltidial plates are frequently retained. Cardinal slopes of both 

 valves broad and not distinctly defined; anterior margin subtruncate 

 and gently sinuate. Hinge plate in brachial valve concave, divided 

 by a deep central concavity, which is supported by a medium 

 septum. Brachidinm consisting of two spiral cones arranged base 

 to base, connected by a V-shaped jugum. 



Whitiieldella nitida Hall (Fig. 123). Atrypa n i t i d a Hall 

 (1852. Pal. N. Y. 2:268, pi. 55) 



Distinguishing characters. Small size; 

 strong convexity of valves, which are 

 nearly equal, pointed incurved beak of 

 pedicle valve; smooth surface except for 

 concentric growth lines and strong 

 wrinkles of growth; often slight sinus 

 near the front of both valves, causing 

 slight frontal emargination. 



Found abundantly in the Clinton lenses 

 and less commonly in the Rochester shale at Niagara. 



This species varies from broadly to narrowly ovate; the thick- 

 ness is frequently greater than the width and coarse thickenings 

 and wrinkles show changes in growth. This may characterize 

 senile individuals. 



Fig. 123 Whitfieldella nitida 



Whitiieldella nitida var. oblata Hall (Fig. 124). Atrypa 

 nitida var. oblata Hall (1852. Pal. N. Y. 2:269, pi. 55) 



Distinguishing characters. Broadly 

 ovate form, angle between cardinal 

 slopes often 90 or more; moder- 

 ately convex valves, subtriangular in 

 cross-section; uniformly rounded 

 front; surface absolutely smooth, very 

 deep muscular impressions. 



Found in the Clinton lenses and the Rochester shale associated 

 with the preceding and usually more abundant. Also at Lockport, 

 etc. (Flail). 



Fig. 124 Whitfieldella nitida var. oblata 



