4(H> PALEONTOLOGY OF NEW-YORK. 



Dorsal valve usually convex in the upper part, concave in the middle 

 by a broad undefined sinus, which, towards the front, often involves 

 the entire width of the valve : beak not incurved. 

 ScKFACK smooth, or with faint concentric lines of growth : shell com- 

 pact and very finely punctate. The shell varies from three-tenths to 

 four-tenths of an inch in length ; the width usually more than three- 

 fourths as much, and sometimes nearly equal to the length. 

 The interior of the ventral valve shows two strong teeth, at some 

 distance below the apex, with strong dental lamellae. The interior of 

 the dorsal valve shows the bases of the crura to be very thick and 

 strong, entirely divided at the centre, and each supporting a thin fila- 

 ment, which becomes broader below and sends off a spur into the ventral 

 cavity ; and thence curving inwards, the outer margins are united and 

 produced along the line of junction in a slender elevated carina, which 

 extends forward in a slender free point. The muscular imprint is oval, 

 and divided along the centre. 



The casts of the interior preserve thie impressions of the features 

 described, and are readily recognized by the slender incurved filling of 

 the rostral cavity. 



This species presents considerable variety in its form and proportions. The 

 dorsal valve is often gibbous in its upper part, with or without a median sinus, 

 which becomes developed below the middle. In other individuals, the gibbous 

 upper part of the valve is marked by a narrow sinus, which becomes broader 

 below ; and in others, the dorsal valve is concave throughout its entire length 

 and breadth, except a slight convexity along the cardinal slopes. The front is 

 sometimes truncate or sinuate, and not unfrequently pointed from the extension 

 of the mesial ridge of the ventral valve, and the corresponding sinus of the dor- 

 sal valve. 



In a single specimen from the Schoharie grit, the dorsal valve is quite flat ; 

 and another from the Corniferous limestone has the ventral valve scarcely snban- 

 gular in the middle, and the dorsal valve regularly and gently convex. 



Geological fonmations and localities- This species occurs in the Schoharie grit, 

 at numerous places in Albany and Schoharie counties, and particularly near Clarks- 

 ville and Schoharie. It is likewise found in the Corniferous limestone at numerous 

 localities, but is usually somewhat rare in this rock in New- York. It occurs in the 

 same horizon in Canada West and in Ohio. 



