180 PALAEONTOLOGY OF NEW YORK. 



valves are the same in form; the sinus in the wing of the right valve is not 

 so deep as in the left valve, liut nuu'ii dreper tlian in the right valve of that 

 s]>eoies. 



The original specimen is iUnstrateil in lignre 14 oC plate xxi, and ligures 17, 

 IS, T.I are right valves occurring in the same jjeds. 



This form is associated with Avicula spinigera of Conrad, wliich is a smaller 

 siiell, less oblique, narrower, and witli a more angidar anterior extremity. 



Formation and localities. In the Clicnumg grou]), at Painted Post, Steuben 

 county, between Corning and Kluiini. and at Chemung Narrows, N. Y. 



LePTODESM.V SUUMARUI, U. Sp, 

 PLATK LXXXIX, FIGS. 5, G. 



SiiF.LL of medium size, suh-rliouiboidal : body narrow, very ()))liquely-ovate ; 

 length soraewh;it K'ss tlian twice the height; margin very broadly curving 

 from the anterior extremity to the base ; basal margin rounded and :dirupt!y 

 recurved at the post-basal extremity. 



Left valve ([uite convex; the greatest convexity about one-third the length 

 of the valve below the a])ex. Right valve unknown. 



Hinge-line straight; length about equal to the height of the shell. 



Beak sub-anterior, obtuse, prominent and arching over the hinge-line. 

 Umbonal region gibbous, subtending an at-ute angle. 



Anterior extremity short, acute, straight above, obli(iue l)elow. Wing 

 small, narrow-triangular, extending to a [Joint below the middle of the length 

 of the shell ; margin concave ; extremity acute, probably spiniibrm. 



Test thin, marked by lamellose, often fasciculate strite, which are more 

 crowded on the anterior side and byssal depression, and more regular and 

 distinct on the wing. 



Interior unknown. 



Tliree .specimens of left valves have res])ectively the following dimensions : 

 length 42, 40 and .'Ki nun., height 24, 27 and 24 mm., and hinge-line about 

 20, 24 and 24 nnu. 



