374 
PALAEONTOLOGY OF NEW YORK. 
Posterior extremity regularly rounded. Cardinal line nearly straight, slightly 
declining posteriorly. The anterior end varies from regularly and narrowly 
rounded to obliquely truncate below and abruptly rounded above. 
Valves regularly convex below and posteriorly, becoming gibbous above and 
in the umbonal region. 
Beaks sub-anterior, large and prominent, inclined forward. Umbonal 
ridge not defined ; shell usually indistinctly flattened from the beak to the 
basal margin, which is often constricted by this depression. 
Surface marked by fine, close concentric striae and by strong rounded or 
sub-angular concentric undulations, which are usually continuous from the 
lunule to the margin of the escutcheon. These undulations are sometimes 
duplicated, or others intercalated on the anterior half of the shell, which are 
thence continuous to the posterior termination. In some older specimens these 
ridges become obsolete on the umbonal slope. Test also marked by fine, dis¬ 
tant radiating striae, which have been observed only on the posterior slope. 
Lunule large but not distinctly limited. Escutcheon, in well-preserved 
specimens, very distinctly defined by a broad depression margined longitud¬ 
inally on each side by a low ridge. Characters of the interior unknown. 
Five specimens measure respectively 34, 38, 40, 45 and 57 mm. in length, 
and 22, 26, 26, 28 and 35 mm. in height. 
This species differs from G. alveata in its more elongate form, more rounded 
posterior extremity, flattened or constricted basal margin, and in having the 
concentric folds usually extending with nearly undiminished strength across 
the umbonal slope. 
Formation and localities. In the arenaceous shales of the Hamilton group, in 
Schoharie county, and common in the softer shales in central and western New 
York. It has also been obtained from the soft shales of this group at Patter¬ 
son’s creek, Virginia, and in the clierty layers above the Corniferous limestone 
at the Falls of the Ohio. 
