200 PALAEONTOLOGY OF NEW YORK. 



joiniii";' the hofly near the po.sterior cml : extremity produced, aoutc, and 

 ])rol»iil)ly extended into a short spine. 



Te.st marked l>y fine stria^ of trrowth whioli are irreguhirly tasciculate. 

 prodncing a sliglit uinkilation of tlie surface, Tlie striiv are hmiellose on 

 the anterior side and regular on the winu'. 



Interior unlvuown. 



The specimen descrilied has a greatest length of aljout 40 mm., heiglit li") 

 mm., and hinge-line •!•"> mm. 



This form difters from L. unibonaium in tlie lesser convexity and greater width 

 of the lower part of the hody ; less ahrujjt depression on the anterior side from 

 the umho; the posterior slope less elevated and defint'd lielow the middle, and 

 a much larger ante-lnssal jtortion of the shell. 



Formation and locality, in the upper part oi' the (.'henumg group, Najioli 

 Centre, Cattaraugus county, N. Y. 



LeI'TODES.M.V X.VVIKORME. 

 IM.ATK .WII, FIG. 15; ANii I'LATK -WUI, Flli. ]. 



Leptodemna nav'ifonm. Hall. Pal. N. V.. vol. v. |it. 1. Phites and Kxplanalidiis : PI. 22. titr I."); 



pi. 28. tif.'. 1. .Ian.. ISSS. 



Shell helow the medium size, rhomboidal ; body very oblique, short-ovate; 

 length nu)re than one-third greater than the height ; anteiior extremity 

 subtruncate ; margin curving to the broadly roinided lia.se; posterior margin 

 slightly proiluced, gently curving forward, nearly vertical. 



Left valve convex below, very gibbous above the middle. Kight valve 

 depressed-convex below, gibbous aliove. 



Hinge-line straight, more than one-third greater than the height of the 

 shell. 



Beaks obtu.se, situated at the anterior fourth of the liinge-liiie. prominent. 

 directed slightly forward. Umbonal region gibbous, sloping abruptly into 

 the wing. 



