Lamellibranchiata. 353 



A right valve, referred to this species, has a length of 32 mm., 

 height 27 mm., and hinge-line, exclusive of the spine, 31 mm. 



This species is larger and more robust than L. longispinurn ; the 

 body is wider, less oblique, and less abruptly recurving at the post- 

 basal extremity; the wing is larger and the greatest depth of the sinus 

 not quite so near the cardinal line; the striae do not recurve so ab- 

 ruptly forward on the posterior slope; the right valve is propor- 

 tionally higher; the wing is less deeply sinuate, and the spiniform 

 extension is shorter. Compared with L. Mortoni, the body of the 

 shell is less oblique and more expanded, while the wing is much 

 smaller and the sinus nearer the hinge-line. 



Formation and locality. In the middle portion of the Chemung 

 group at Painted Post, Steuben county, N. Y. 



Leptodesma Agassizi. 



Leptodesma Agassizi, Hall. Pal. N. Y., vol. v, pt. 1. Unptiblis7ied. 



Shell of medium size, sub-rhomboidal; body broadly ovate, oblique 

 to the hinge-line at an angle of from 50° to 55°; length nearly one- 

 third greater than the height; anterior margin broadly rounded 

 below the byssal sinus; ventral margin regularly curved ; more 

 abruptly curved at the post-basal extremity. Left valve moderately 

 convex below, more convex above and gibbous on the umbo. Right 

 valve less convex. Hinge-line straight, about equal to the height 

 of the valve. Beaks sub-anterior, scarcely directed forward, 

 prominent, rising a little above the hinge-line. Umbonal region 

 moderately gibbous, subtending an acute angle. Anterior end 

 large, obtuse, truncate above and curving below into the byssal 

 sinus, limited by an oblique depression which reaches to near the 

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

 below the middle; margin oblique below and abruptly concave just 

 beneath the cardinal line, recurving and produced into a short spini- 

 form extension. The right valve is less convex, the wing larger 

 and more extended along the posterior slope. 



Test thin, marked by fine, distant, lamellose striae with inter- 

 mediate finer lines. The stronger striae are more conspicuous on 

 the anterior. The partial casts are nearly smooth. 



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

 base of spine, 15 mm. Another example has a length of 26 mm., 

 height 19 mm., hinge to base of spine, 15 mm. 



[Sen. Doc. No. 38.] 45 



