4©« PALAEONTOLOGY OF NEW-YORK. 



m Centroiiella iuipressa. 



PLATE LXI. 



Centrontlla impretsa : Hall, Fourteenth Heport on the State Cabinet, p. 102. 18G2. 

 " " Idem, Fifteenth Report on the State Cabinet, p. ICO- 101. 1862. 



• Shell subovate in outline, sometimes extended in front, rarely sub- 

 sinuate ; width and length about as seven to nine. 

 Ventral valve convex, prominent or subangular along the middle and 

 abruptly curving to the sides ; longitudinal outline slightly arched, 

 sometimes nearly straight : beak nearly erect and truncated by a 

 rounded foramen, which is limited below by deltidial plates. 

 Dorsal valve much shorter than the vetftral, in the upper part convex 

 at the sides, and flattened or depressed in the middle and towards the 

 front, which is much produced and curves downwards to occupy the 

 sinuous outline of the ventral valve. Sometimes the dorsal valve is 

 nearly flat, with an impressed line down the centre, and sometimes 

 flattened in the upper part and convex in the lower part : beak not 

 incurved. 

 Surface with fine concentric strioe and nearly obsolete remains of very 

 fine radiating strioe, which arQ visible only under a lens. Shell com- 

 pact ; texture punctate. 



The interior of the ventral valve shows a very strong tooth on each 

 side at the base of the fissure, and nearly one-third the length of the 

 valve from the apex. The interior of the dorsal valve shows the dental 

 sockets and a remarkably strong thickened hinge-plate, which is concave 

 in the centre, the margins extended and inclosing an oval muscular area, 

 which is divided by a low longitudinal septum. Outside of the muscular 

 area there is a low longitudinal ridge extending to the anterior margin 

 of the shell. [In these features, it resembles, but is very distinct from, 

 the interior of the dorsal valve of C. glans-fagea-l The crura have not 

 been observed. 



