354 Thirty-fieth Keport ok the State Museum. 



This species bears a general resemblance to L. spinigerum, with 

 which it is associated, but the body is more erect and much wider 

 below; the wing is somewhat wider, the spine shorter, and the striae 

 less strongly marked. 



Formation and locality. In the middle portion of the Chemung 

 group, on the Chemung river, between Elmira and Waverly, N. Y 



Leptodesma protextum. 



Avicula protexta, Conrad. Jour. Acad. Nat. Sci. , Phila., vol. viii, p. 238, pi. 



12, fig. 6. 1842. 

 Pterinea protexta (Conrad), S. A. Miller. Cat. Amer. Pal. Foss., p. 202. 



1877. 

 Leptodesma protextum (Conrad), Hall. Pal. N. Y., vol. v, pt. 1. Plates and 



Explanations : PI. 21, figs. 22, 23. Jan., 1883. 



Shell small, sub-rhomboidal ; body elongate-ovate, very oblique ; 

 length nearly twice as great as the height ; anterior margin below 

 the byssal sinus, oblique ; base broadly rounded and abruptly re- 

 curved on the post-basal extremity. Valves sub-equally convex ; 

 the greatest convexity is above the middle of the length, becoming 

 less toward the posterior side, which is somewhat depressed. The 

 right valve is apparently a little smaller than the left. Hinge-line 

 straight, about equal to the length of the shell. Beaks near the an- 

 terior end of the valve obtuse, rounded, prominent, inclined for- 

 ward. Umbonal region narrow, gibbous for a short space below the 

 hinge, subtending an acute angle. Anterior end scarcely auriculate, 

 abruptly rounded in the left valve, pointed in the right valve, limited 

 by a broad, undefined depression, which is less marked in the right 

 valve. Byssal sinus shallow. Wing narrow-triangular, joining the 

 body near the posterior extremity of the valve, defined by the di- 

 rection of the stria3 ; margin deeply concave, the greatest concavity 

 just beneath the hinge-line ; extremity produced into a sharp spine 

 which extends nearly as far as the posterior margin of the shell. 



Test thin, marked by concentric striae of growth, which are 

 crowded into fascicles at irregular intervals, giving a somewhat 

 wrinkled or undulating surface. On the anterior side the striae are 

 crowded and lamellose, while they are quite regular and closely ar- 

 ranged over the wing. On the external shell the striae are elevated 

 into regular, equi-distant lamellae. In the exfoliated shell, or partial 

 casts, the surface presents obscure or obsolescent radii which appear 

 to belong to the intimate shell-structure, and which are not shown 

 on well-preserved specimens. 



