370 PALEONTOLOGY OF NEW YORK. 



c. undulata. 

 Grammysia alveata. 



PLATE LVir, FIGS. 1,2; and PLATE LX, FIGS. 1-11. 



Ptki'idtmia alveata, Conrad. Geol. Surv. N. Y., Ann. Rep., p. 53. 1841. 



Orammj/xia edveata (Conrad), Hail. Prelim. Notice Lainellibranchiata, 2, p. 55. 1870. 



" " " " Pal. N. Y., vol. V, pt. 1. Plates and Explanations: PI. 57, figs. 1, 2; 



pi. 60, figs. 1-11. 1883. 



Shell large, ovate-rhomboidal ; length more than one-third greater than the 

 height ; basal margin regularly curved, abruptly recurving at the post-inferior 

 extremity and broadly truncate above. Cardinal line short, nearly straight ; 

 sub-alate at the post-cardinal extremity. Anterior end short, abruptly 

 rounded. 



Valves convex below and posteriorly, ventricose above. 



Beaks sub-anterior, large, prominent, strongly incurved over the hinge- 

 line. Umbonal slope rounded or sub-angular, extending to the post-inferior 

 extremity. Cardinal slope concave. Cincture obsolete. 



Entire surface marked by fine concentric striae and, anterior to the umbonal 

 slope, by strong, angular, concentric folds or more gentle undulations. 



Hinge marked by a thickened plate bearing an obscure fold which is not 

 so strongly marked as in several of the preceding species. 



Two specimens measure respectively 68 and 88 mm. in length, and 47 and 

 62 mm. in height. A specimen preserving both valves has a length of 58 

 mm., height 42 mm. and depth of 36 mm. Three vertically compressed indi- 

 viduals of large size measure respectively 84, 94 and 104 mm, in length, 34, 

 58 and 39 mm. in height, and 52, 66 and 57 mm. in the depth of both valves. 



This species closely resembles G. Urate, and that one may be only a modifi- 

 cation and smaller form of this species. In general it may be distinguished by 

 its fewer concentric folds and their entire absence on the posterior slope. It 

 differs from G. arcuata in its more erect form and the absence of the concentric 

 folds on the posterior slope. 



Formation and localities. Abundant in the arenaceous shales of the Hamilton 



