120 PALjEONTOLOGY of new YORK. 



A("TlNOI'Ti:i;iA TKNUISTllIATA, II. sp. 

 ri. \ TK I.\\\I\ . K1(;S. 5, G. 



Shkll small, .^uli-quadrate ; boily short-ovatr, ()l)li((U(' at an angle of 60" with 

 the hinge ; length ahout (»nc-lirtli greater than the height; anterior margin 

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

 base; posterior margin broadly cnrvcd. 



Left valve regularly convex hclnw the middle, gibbous du the umbo. 

 Kight valve smaller, nearly equall}- 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. Und)()nnl region gibl)ons and ample, subtending 

 nearly a right angle. 



Ear small, defined by a distinct siinis and byssal tlepression, roundeil at 

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

 its length, not strongly limite(l ; margin gently concave; extremity acute. 



Test thin, marked l*}' fine concentric stria\ and lililnrm radintions with 

 wider interspaces, which are marked by extremely slender intermediate 

 striie. On the anterior side the radiating lines are liner, ami on the cardinal 

 margin of tlie wing they are stronger. The right valve is nnirkiMJ oidy liy 

 elevated concentric striiv, and the wing liy line radiations, which become 

 stronger toward the cardinal margin. 



One S2)ecinien has a length of 10 mm., height I'l nnn.. and hinge-line 

 iJ nnn. 



This species is distinguished from A. perstrialis by its erect and more nearl\- 

 (juadrate form, shorter ear, less extemlec] wing with less concave margin, ami 

 more distant, finer strife. 



Formation and locality. In the lower part of tlu' (Jhenunig gioup at 

 Ithaca, N. Y. 



