118 PALEONTOLOGY OF NEW YORK. 



Kijrlit valve convex on tlie uiulx), liecoming depressed below the middle of 

 the leiifitli. I.elt \;dve unknown. 



Ilinjie-line straiglit, less than the length of the shell. 



Beak anterior, acute, moderately prominent, rndiomil icuion scarcely' 

 gibbons, subtendini;' an acute au,<;k'. 



Wini:- narrow, triangular, elongate, extending nearly to the posterior end 

 oltlie shell; margin concave; extremity abruptly acute. 



Test thin, marked by elevated, sub-imbricating concentric bands, piodiued 

 bv the lamello.se character of the stria*. Crossing these bands, on the 

 posterior half of the shell, are interrupted radiating lines, which indicate the 

 presence of radii upon tiu' original surface. The wing is marked in a 

 similar manner by strong liands. w hicli ])ass over the hinge-margin, and it 

 also shows two or three interrupted radiating lines, like tiiose on the poste- 

 rior part of the body of the shell. 



Interior unknown. 



The specimen described has a lengtli of 11 mm., height 12 mm., and 

 hinge-line Id mm. 



This .species is allieil to A. decussata and A. Boijdi, having the wing charac- 

 teristic of the former; but it is a proportionally shorter foi'm. and the 

 surface nuirkings are ((uite unlike the right \alveof any of the forms at present 

 known. 



Formation and locality. In shales of the ILimilton group, on the shores of 

 Canandaigua lake, N. Y. 



Al'Tl.NOl'TEra.V rEUSTllIALlS. 

 IM.ATK XXm, KIti.S. i, 7; and I»r,.\TE LXXXIV, KIC \>. 



AitiiuipUriii pirxtiialix, II.m.t.. Pal. N. V., vul. \, jit. 1. Plates ami I'",Niil.ni:iliiMis : PI. •.':>. liirs. J, 7. 



Jan., ISS:^. 



SiiKi.i. small, sub-rhond)oidal ; body sub-ovate, axis slightly curved and ol.li(pie 

 at an angle of about 45" with the liinge; height al)out three-fourths the 

 length of the l)ody; basal margin l)roadly rounded; posterior margin aluuptly 

 curved. 



