134 PALJEONTOLOGY OF NEW YORK. 



margin liroadlv curved ; posterior extremity iilniijitly or sul)-aiigularl3^ 

 recurved. 



Li'l't valve convex, giljltous on tlie iimlio and obtusely sub-anuidar along 

 ilir post-cardinal slope. Rigiit valve less convex, somewhat smaller than the 

 left. 



Hinge-line straight; length about two-thirds the length ol" the valve. 



Beak near the anterior fourth of tli(> length ol' the hinge, directed forward ; 

 beak of left valve prominent, acute, incurved over the hinge-line; beak of 

 right valve obtuse, and not elevated. Tmljonal region of the left valve 

 gibbous and angular, of the right valve convex, subtending a very acute 

 angle. 



Anterior end small; margin rounded ; extremity at'ute. Byssal depre.s.sion 

 iililiiiue, directed backward. Wing naiTow, triangular, joining the body at the 

 posterior end; margin obli([iii'ly tnuieate, scarcely concave; extremity 

 apparently not produced. 



Surface marked by fine radiating striie, which are more conspicuous on the 

 body of the valve and upon the wing, and are very obscure on the anterior 

 end; these are crosseil l)y fine concentric stria> of growth which, at irregular 

 intervals, are raised into fa.scicles, usuall}- more con.spicuous on the umbo 

 and post-cardinal slope. On the right valve the radii are usually obsolete. 



Interior unknown, exce])t the usual linear groove along the hinge-line. 



A large left valve ha-< a length of 4(1 mm., height 2(J mm., and hingedine 

 28 mm. Another is 38 mm. in length, height 20 mm., and hinge-line 27 mm. 



In this species the Ijody is more ohlicpie than in P. Salamanca ; \\iv angularity 

 of the left valve is in a direct line down the middle of the body ; the shell 

 is comi)aratively narrower; the extremity of the wing is not produced; the 

 byssal depression is more obli(iue. The want of striaa on the anterior end and 

 their ob.solesceiu'e on the right valve are marked characters. It is propoitionallv 

 much longer than P. Sao, with which it is associated. 



Formation and locality. In the lower part of a conglomerate of the ("heniung 

 group at Panama, Chautauqua county, N. Y. 



