200 PALEONTOLOGY OF NEW YORK. 



joining the body near the posterior end; extremity produced, acute, and 

 probably extended into a short spine. 



Tes1 marked by fine striae of growth which are irregularly fasciculate, 

 producing a slight undulation of the surface. The striae are lamellose on 

 the anterior side and regular on the wing. 



Interior unknown. 



The specimen described bas a greatest length of about 40 nun., height 2o 

 mm., and hinge-line 35 nun. 



This form differs from L. umbonatum in the lesser convexity and greater width 

 of the lower part of the body : less abrupt depression on the anterior side from 

 the umbo; the posterior slope less elevated and defined below the middle, and 

 a much larger ante-byssal portion of the shell. 



Formation and locality. In the upper part of the Chemung group, Napoli 

 Centre. Cattaraugus county. X. Y. 



Leptodesma naviforme. 



1'I.ATi; XXII, I'll. 15 ; and PLATE XXIII, FIG. 1. 



Leptodesma naviforme, Hall. Pal. N. V.. vol. v. ]it. I. Plates and Explanations: PI. 22. fig, 15; 



pi. 23, fig. 1. Jan.. 1883. 



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

 length more than one-third greater than the height ; anterior extremity 

 subtruncate; margin curving to the broadly rounded base; posterior margin 

 slightly produced, gently curving forward, nearly vertical. 



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

 depressed-convex below, gibbous above. 



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



shell. 



Beaks obtuse, situated at the anterior fourth of the hinge-line, prominent, 

 directed slightly forward, Umhonal region gibbous, sloping abruptly into 



the wing. 



