402 
PALEONTOLOGY OF NEW-YORK. 
I 
Centronella impressa. 
PLATE LXI. 
Centronella impressa : Hall, Fourteenth Report on the State Cabinet, p. 102. 1862. 
“ « Idem, Fifteenth Report on the State Cabinet, p. 160-161. 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 ventral, 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 striae and nearly obsolete remains of very 
fine radiating striae, which are 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 clmcave 
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.] The crura have not 
been observed. 
