LAMELLIBRANCHIATA. 183 



byssal siiiu.s, limited by an oblique depression which reaches to near the 

 middle of the length of the valve. Wing small, joining the body ))elow the 

 middle ; margin oblique below and abruptly concave just beneath the cardinal 

 line, recurving and produced into ii short spiniform extension. The right 

 valve is less convex, the wing larger and more extended along the posteiior 

 slope. 



Test thin, mai'ked by fine, distant, lamellose striae with intermediate finer 

 lines. The stronger strite are more conspicuous on the anterior. The 

 partial casts are nearly smooth. 



Interior unknown. 



One specimen has a length of 29 mm., height 23 mm., hinge, to base of .spine, 

 li") mm. Another example has a length of 26 mm., height 10 mm., hinge, to 

 base of spine, 15 mm. 



This species Ijears a general resemblance to L. spinigeruni, witli which it is 

 sometimes associated, l^ut the l)od\- 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. 



PLATE XXI, FIGS 2-2, 23 



Avieula protextUy Cokrad. Jour. Aoad. Nat. .Sci., Pliila.. vi>l. viii. p. 238, pi. 12, fig'. 6. 1842. 

 Pterinea prolexta (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 : PL 21, figs. 22, 



28. 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 abrui)tly recurved 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. 



