Lamellibranchiata. 347 



broadly curving from the byssal sinus ; post-basal margin produced 

 and abruptly recurved. Left valve very convex, gibbous in the 

 middle and above. Eight valve unknown. Hinge- line straight, 

 about equal to the height of the valve. Beak sub-anterior, directed 

 forward, acute, prominent and arching over the hinge. Umbona] 

 region narrow, ventricose, well-defined, subtending an acute angle. 

 Ear large, bending downward, marked by a strong oblique fold, 

 limited by a broad, vertical byssal depression and shallow sinus ; 

 margin convex ; extremity apparently obtuse. Wing joining the 

 body above the posterior extremity limited by the nearly vertical 

 post-umbonal slope which makes a more or less marked sulcus ; 

 margin concave ; extremity produced, acute. 



Surface marked by fine striae of growth which are somewhat 

 regularly lamellose on the body of the valve. 



A left valve has a length of 29 mm., height 22 mm., hinge-line 

 21 mm. 



This species differs from L. Chemungensis in its proportionally 

 greater length, narrower umbo, more convex and oblique left valve, 

 and shorter wing. 



Formation and locality. In a sandstone and conglomerate of the 

 Chemung group near Panama, N. Y. 



LEPTODESMA, Hall. 



The species arranged under the generic designation of Leptodesma 

 present a very remarkable assemblage of forms, beginning (as here 

 arranged) with those resembling Avioula and passing through various 

 phases in form, proportion, etc., until the wing becomes nearly obso- 

 lete, the byssal sinus obscure, and the anterior end rounded, and the 

 entire shell assuming the form of Mytilus or Modiola, Sangulno- 

 lites, Modiomorpha and allied genera. For convenience the species 

 have been arranged in natural groups in the following order; a, 

 spinifera ; b, umoonata ; c, rostrata; d, patulata ; e, arcoidea ; 

 f mytiloidea. 



Section a, spinifera. 

 Leptodesma Marcellense. 



Leptodesma Marcellense, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explana- 

 tions : PI. 17, fig. 12. Jan., 1883. 



Shell small, sub-rhomboidal ; body obliquely ovate ; height about 

 two-thirds the greatest length ; margin regularly rounded, extended 



