LAMELLIBRANCHIATA. 73 



Ears sub-equal, triangular, undefined; margins concave. Anterior ear 

 limited by a sulcus and shallow sinus; extremity acute. In the right 

 valve sub-triangular; margin rounded, defined by an abrupt sulcus and a 

 deep, angular byssal notch, which encroaches on the body of the valve. In 

 the posterior ear the extremity is obtuse in the left valve, somewhat more 

 pointed in the right valve. 



Test thin, marked by numerous fasciculate rays, which increase by inter- 

 stitial addition, witli interspaces varying from nearly equal to several times 

 the width of the rays; cancellated and sometimes crenulated by elevated, 

 lamellose stria? of growth ; and by from three to ten strong, concentric undu- 

 lations, which are rounded upon the summits. The markings on the cars 

 are similar to those on the body of the shell; the rays on the anterior ear 

 are sometimes obscured by the stria? of growth. 



Interior not known. Ligamental area narrow. 



A left valve has a length of 51 mm., height 36 mm., hinge-line 36 mm. 

 A right valve has a length of 44 mm., height 37 mm., hinge-line 35 mm. 

 A young specimen has the following dimensions : length 23 mm., height 

 19 mm., hinge-line 20 mm. 



This species is distinctly characterized by the strong, concentric wrinkles, 

 the deep byssal sinus of the right valve, and by the form of the shell. Speci- 

 mens vary in the frequency and elevation of the radii, and in the number and 

 prominence of the concentric undulations. 



The specimens are casts of the interior, with scarcely any remaining test. 

 The surface markings are therefore subdued, and the concentric stria? are often 

 not at all preserved. 



This species is widely distributed. It has been found in the Corniferous 

 limestone at a single locality ; and in the soft shales of the Hamilton group 

 throughout the western portion of the State. 



Formations and localities. In the Corniferous limestone at Clarence Hollow, 

 Erie comity : in shales of the Hamilton group at numerous localities in Central 

 and Western New York. 

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