NUCLEOSPIRA OP THE HAMILTON GROUP. 
side of the valve often extends from beak to base. Dorsal valve 
furnished with a strong spatulate cardinal process, which, rising verti¬ 
cally from the cardinal margin, is closely grasped at its base by the car¬ 
dinal teeth of the other valve; and thence bending abruptly upwards, 
and expanding, is projected into the cavity of the opposite beak, lying 
close upon the underside of the false area. This process is grooved or 
depressed in the centre of the upper side, so as to leave' between it and 
the arch of the ventral beak a narrow space for the passage of a pedicle, 
a minute foramen being sometimes observed in the beak for its pro¬ 
trusion. From the sides of this process, above the junction of the teeth 
of the opposite valve, and at the point where it bends upwards, originate 
the crural processes which support the spires. A deep cavity beneath 
the cardinal process extends to the dorsal beak, where originates a thin 
elevated septum often extending to the base of the shell. Muscular 
impressions of the dorsal valve occupy a narrow oval space; those of 
the ventral valve broader, flabelliform. 
Under this genus I have described N. ventricosa, N. elegans and N. concentrica from the 
Lower Ilelderberg rocks; N. pisiformis (Orthis pisum, Pal. New-York, Yol. ii, p. 450), 
from the Niagara group; and N. concinna ( Atrypa concinna, Rep. of the Fourth Geol. Dis¬ 
trict of New-York, 1848, p. 200, f. 3), which are the species known to me at the beginning 
of 1858. 
Nucleospira concinna. 
PLATE XIV. 
Jtlrypa concinna : Hat/l, Geological Report Fourth District New-York,, p. 200, f. 3. 1843. 
Nucleospira concinna : Hall, Twelfth Report on the State Cabinet, p.25 and p.26. 1859. 
Shell depressed, subspheroidal, nearly circular in outline, the width 
being usually a little greater than the length; valves subequal. 
Ventral valve regularly convex, the greatest convexity a little above 
the middle, and curving regularly to the sides and front; umbo pro¬ 
minent, the beak neatly pointed and incurved over the apex of the 
dorsal valve, leaving a space between which sometimes exposes the 
narrow area. There is usually a narrow depressed line from the beak 
to the base of the valve ; but this is sometimes partially absent, or so 
faint as not to be readily observed. 
