LAMELLIBRANCHIATA. 225 



Hinge-line straight ; length a little more than the greatest length of the 

 valve. 



Beak sub-anterior, directed forward, acute, not rising much above the 

 hinge. Umboual angle acute. 



Anterior end narrow, elongate, very acute, limited by a shallow" byssal 

 depression. Wing large, joining the body near the base ; margin broadly and 

 symmetrically concave ; extremity produced into a spiniform extension reach- 

 ing beyond the posterior extent of the shell. 



Test marked by fine concentric striae which are regular on the posterior 

 part of the body and the wing, and very crowded and fasciculate on the 

 anterior. 



Interior unknown. 



A large left valve has a length of 60 mm., height 37 mm., hinge-line 

 62 mm. A slightly larger, imperfect specimen has been found, the body of 

 which, near the posterior extremity, has a greatest width of from 25 to 28 

 mm., and measures from 12 to 14 mm. in the narx'ow part between the beak 

 and the byssal sinus. 



This species differs from L. Pelops in the longer and more ei'ect body, more 

 attenuate anterior extremity, more extended wing and deeper sinus in the 

 margin. 



Formation and localities. In the middle beds of the Chemung group at 

 Charleston, and near Mansfield, Tioga county. Pa. 



Section d, patulata. 

 Leptodesma flaccidum, n. sp. 



PLATE XCI, FIG. 9. 



Shell of medium size, rhomboid-ovate, spatulate below ; body oblique at an 

 angle of about 45° to the hinge-line ; length about one-third greater than 

 the height ; ante-byssal margin subtruncate, oblique, a little concave below, 

 curving into the broad ventral margin ; posterior margin abruptly rounded 



and recurved. 



29 



