LAMELLIBRANCHIATA. 375 



Grammysia subarcuata. 



PLATE LXI, FIGS. 10-2> ; and PLATE XCIII, FIG. 26. 



Gh-ammysta imtiarcicata. Hall. Prelim. Notice Lamelllbranchiata, 2, p. 61. 1870. 



Grammysia (Leptodomus f) subarcuatu, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 61, 



figs. 10-22. 1883. 



Shell of medium size, sub-ovate, widest at the posterior end; length more 

 than one-third greater than the height; basal margin regularly convex, 

 sometimes straight or constricted toward the anterior end. Posterior 

 extremity rounded, or obtusely sub-angular, often sub-truncate above. Car- 

 dinal line nearly straight. " Anterior end short, abruptly rounded below the 

 lunule. 



Valves moderately convex in the lower and posterior part, becoming gib- 

 bous in the middle and umbonal region. 



Beaks sub-anterior, prominent, inclining forward and strongly incurved. 

 Umbonal slope rounded. Post-cardinal slope flattened or concave, some- 

 times limited by an elevation, at which line the concentric undulations ter- 

 minate. Anterior to the middle of the shell, extending from the beak to 

 the basal margin, there is a depression or cincture which often gives a slight 

 undulation to the concentric folds and a constriction to the margin. 



Surface marked by fine, close concentric striae and by strong sub-angular 

 concentric ridges or folds, which are frequently duplicate posterior to the 

 cincture. The surface is also marked by fine, radiating, pustulose striae, 

 which are often very conspicuous. Interior unknown. 



Three specimens measure respectively 36, 38 and 48 mm. in length, and 

 20, 23 and 33 mm. in height. The largest specimen observed has a length 

 of 70 mm. 



This species resembles G. arcmta in its general aspect, but differs in its more 

 elongate form, more pointed posterior end, and the cincture is much more 

 strongly marked. 



Formation and localities. In the arenaceous layers of the Chemung group, at 

 Ithaca, near Elmira, Portville and Chemung, N. Y. 



