120 PALjEdNTOLOGY OF NEW YORK. 



A.CTINOPTERIA TENDISTRIATA, ll. sp. 

 I'l. VI I, LXXXIV, PIGS. 5, 6. 



Shell small, sub-quadrate; bodj short-ovate, oblique at an angle of 60° with 

 the binge; length about one-fifth greater than the height ; anterior margin 

 below the sinus nearly vertical, and then making a broad curve along the 

 base; posterior margin broadly curved. 



Left valve regularly convex below the middle, gibbous on the umbo. 

 1 tight valve smaller, nearly equally convex. 



Hinge-line straight, a little greater than the height of the valve. 



Beaks at about the anterior third of the hinge-line, prominent, obtuse, 

 rising above the hinge. Umbonal region gibbous and ample, subtending 

 nearly a right angle. 



Ear small, defined by a distinct sinus and byssal depression, rounded at 

 the extremity. Wing small, triangular, joining the body below the middle of 

 its length, not strongly limited ; margin gently concave; extremity acute. 



Test thin, marked by line concentric striae, and filiform radiations with 

 wider interspaces, which are marked by extremely slender intermediate 

 stria>. On the anterior side the radiating lines are finer, and on the cardinal 

 margin of the wing they are stronger. The right valve is marked only by 

 elevated concentric stria', and the wing by line radiations, which become 

 stronger toward the cardinal margin. 



One specimen lias a length of 16 mm., height 13 mm., and hinge-line 

 14 mm. 



This species is distinguished from A. perstrialis by its erect and more nearly 

 quadrate form, shorter ear, less extended wing with less concave margin, and 

 more distant, liner striae. 



Forma/ion and local//;/. In the lower part of the Chemung group at 

 Ithaca, N. Y. 



