362 PALAEONTOLOGY OF NEW YORK. 



Grammysia magna. 



PLATE LVI, FIGS. 4-7 ; AND PLATE LVII, FIG. 9. 



Omiainytia inagiia. Hall. Prelim. Notice Lamellibranchiata, 2, p. 50. 1870. 



In part " " " Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 56, figs. 2-8; jil. 57, 



fife's. 7-10. 1883. 



Shell large, broadly ovate or sub-rhomboidal ; length more than one-third 

 greater than the height; basal margin gently curving except near the 

 middle, where it is constricted ; post-basal extremity abruptly rounded, with 

 the margin sub-truncate above. Cardinal line straight, extending more than 

 half the length of the shell, declining posteriorly. Anterior end short and 

 abruptly rounded below the lunule. 



Valves regularly convex on the basal and posterior portion, becoming gib- 

 bous in the middle and above. 



Beaks sub-anterior, large, prominent and incurved. Cincture in the right 

 valve, a distinct ridge, margined by a depression posterior to it ; in the left 

 valve there is a corresponding depression. 



Surface marked by fine concentric striae, which become fasciculate, pro- 

 ducing folds or undulations which are conspicuous on the anterior and umbo- 

 nal portion of the shell, and usually obsolete on the posterior portion. The 

 concentric undulations in crossing the ridge near the beak appear like trans- 

 verse nodes. Well-preserved specimens show fine radiating striae crossing 

 the surface, especially in the upper portion. Interior essentially unknown. 



Four specimens measure respectively 72, 82, 98 and 100 mm. in length, 

 and 45, 48, 65 and 65 mm. in height. 



This is one of the large forms of the genus, and is closely allied to G. nodo- 

 costata, from which it differs principally in the form and structure of the oblique 

 cincture, which is a simple fold on the right valve and a furrow on the left, and 

 never divided into nodose ridges or accompanied by lines of nodes on the sides, 

 as in that one. The oblique furrow is often exaggerated by the crushing of 

 the shell, the folding taking place along that line. 



Notwithstanding these differences, which are sufficiently well marked in 



