LAMELLIBRANCHIATA. 227 



somewhat prominent. Umbonal region moderately gibbous, not strongly 

 defined. Umbonal angle acute. 



Anterior end large, produced into a nasute extension. Wing large, 

 joining the shell at the posterior extremity ; margin scarcely concave, 

 abruptly produced along the hinge-line. 



Test marked by fine lines of growth which are irregularly fasciculate, 

 sometimes appearing as equidistant, sharply elevated lamella? ; crowded on 

 the anterior; more regular over the wing and abruptly recurving just 

 beneath the hinge. 



The hinge is marked by one or two narrow, parallel, longitudinal grooves. 



A specimen of medium size has a length of 40 mm., height 26 mm., and 

 hinge-line 28 mm. A similar right valve has a length of 36 mm., height 

 24 mm., and hinge-line 27 mm. 



This species differs from L. flaccidum in its much larger wing, which is more 

 extended along the body of the shell. It is less convex and the umbo less 

 gibbous than in any of the preceding species. 



Formation and locality. In the upper part of the Chemung group at Mans- 

 field, Tioga county, Pa. 



Leptodesma complanatum. 



PLATE XXII, FIG. 2 



Leptodesma complanatum, Hall. Pal. N. Y.. vol. v, pt. 1. Plates and Explanations: PI. 22. fig-. 2. 



Jan., 1883. 



Shell of medium size, obliquely semi-elliptical ; body sub-ovate, moderately 

 oblique ; length one-third greater than the height ; ante-byssal margin sub- 

 truncate, gradually curving into the broad ventral margin ; posterior margin 

 abruptly rounded. 



Left valve depressed-convex, a little gibbous on the umbo. Right valve 

 unknown. ' 



Hinge-line straight ; length less than the height of the shell. 



Beak acute, incurved, directed forward, moderately prominent. Umbonal 



