JjAMELLIBKANCHIATA. 387 



below . It is characterized by the spatulate posterior extension, which 

 is more extreme than in any other species here described. 



Formation and locality. In the Chemung group, Lawrenceville, 

 Tioga county, Pa. 



Leptodesma patulum. 



Leptodesma patulum, Hall. Pal. N. Y., vol. v, pt. 1. Unpublished. 



Shell above the medium size, sub-rhomboidal ; body oblique, elongate, 

 sub-ovate; length slightly more than one-third greater than the 

 height ; ante-byssal margin oblique, gradually curving to the ventral 

 margin which is nearly straight ; posterior margin broadly curved. 

 Valves moderately convex below, scarcely gibbous above, similar in 

 form ; the right valve being a little smaller and less convex than the 

 left. Hinge-line straight, somewhat longer than the height of the 

 shell. Beaks at about the anterior third of the hinge-line, directed 

 forward, somewhat prominent. Umbonal region moderately gib- 

 bous, 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 fascicu- 

 late, sometimes appearing as equi-distant, sharply elevated lamellas ; 

 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 Mansfield, Tioga county, Pa. 



Leptodesma complanatum. 



Leptodesma complanatum, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Ex- 

 planations : 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 



