LAMELLIBRANCHIATA. 377 



sub-angular, truncate above. Cardinal line straight, nearly two-thirds the 

 length of the shell. Anterior end short, regularly rounded. 



Valves regularly convex below and posteriorly, becoming moderately 

 gibbous in the middle and above. 



Beaks sub-anterior, not prominent, incurved. Umbonal slope angular, 

 extending to the posterior extremity, with a distinct fold above the middle 

 of the cardinal slope, and only slightly divergent from the cardinal line. 



Surface of the cast marked by strong concentric folds, which are angular 

 on the anterior end, but become broader and rounded as they approach the 

 umbonal ridge, beyond which they are obsolete. 



Ligamental area deeply marked. Other characters of the interior un- 

 known. 



The specimen described has a length of 33 mm. and a height of 18 mm. 



This species differs from G. precursor in being less ventricose, in having 

 stronger and less numerous concentric folds, and a more defined cincture ; 

 and from G. arcuala in the distinct umbonal ridge, and much stronger post- 

 cardinal fold. 



Formation and localitij. In the Upper Helderberg limestones, at Clarence 

 Hollow, Erie county, N. Y. 



Gkammysia constricta. 

 i'late lix, figs. 13-30, (4, 5?) ; and platk lxxvm, figs. 26, 27. 



Qrammysia (L-^odomasf) coHStrieta, Hall. Prelim. Notice Lamellibranchiata, 2, p. 58. 1870. 



I. <> " •< Pal. N. Y., vol. V, pt. 1. Plates and Explanations : PI. 59, 



figs. 13-20 1883. 

 Pholadella constricta. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : PI. 78, fi{,'-s. 26, 27. 1883. 



Shell small, or below the medium size, narrowly ovate, widest in front ; length 

 nearly twice the greatest height ; basal margin gently curving, with a dis- 

 tinct sinus a little anterior to the middle. Posterior extremity sub-cuneate, 

 narrowly rounded. Cardinal line essentially straight, somewhat rapidly 

 declining toward the posterior end. Anterior end short, rapidly declining 

 from the beaks, sub-truncate above and rounded below. 



