374 PALEONTOLOGY OF NEW YORK. 



Posterior extremity regularly rounded. Cardinal line nearly straight, slightly 

 declining posteriorly. The anterior end varies from regularly and narrowly 

 rounded to obliquelj' truncate below and abruptly rounded above. 



Valves regularly convex below and posteriorly, becoming gibbous above and 

 in the umbonal region. 



IJeaks 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 strife 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 li<ilf 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 arenjiceous 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 cherty layers above the Corniferous limestone 

 at the Falls of the Ohio. 



