LOWER HELDERBERG ROCKS 245 



PLATE CI. 

 Fig. 2 a, 6, c. Dorsal, ventral, and profile views of a specimen presenting the ordinary form 

 of this species in the Oriskany sandstone. 



Fig. 2 d, e. Dorsal and ventral views of a cast of this species. 



Fig. 2 f. A specimen preserving part of the shell and the vascular impressions near the 

 margin. 



Fig. 2 g. A cast of the ventral valve showing the muscular and vascular impressions. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 



group : Hudson; and in the Oriskany sandstone, Helderberg mountains; Schoharie; 



Cumberland, Maryland, and other places. 



Genus Leptoccelia ( n. g.). 



For description and illustration of this genus, see the same under Oriskany 



sandstone. 



Leptoccelia concava. 



Plate XXXVIII. Fia. 1-7. 

 Shell ovate or suborbicular. Ventral valve convex, elevated along the 

 middle into a mesial prominence, which extends to the umbo : beak 

 small, incurved beyond the hinge line. Dorsal valve flattened near the 

 lateral margins, depressed in the middle, forming a shallow undefined 

 sinus which is deeper in the centre than at the front, and rapidly di- 

 minishes towards the umbo : beak straight; area, or false area, small; 

 foramen triangular and extending to the apex of the beak, sometimes 

 closed below by a deltidium. 

 Surface marked by fourteen to seventeen striaB, which sometimes bifur- 

 cate : the one on the mesial fold is generally smaller than the others, 

 giving a slightly grooved appearance along its centre quite to the beak. 

 The sinus in the dorsal valve of this shell widens so rapidly from the beak to- 

 wards the front, and. is so much deeper in the centre than near the beak and at the 

 front of the shell, that it gives in some instances a marked concavity, to this valve. 

 This species may be considered a representative of Terebrattda duboisi of De 

 Vekneuil (Geologic de la Russie, PI. x, f. 16); from which it differs in its more 

 rounded outline, and in the greater concavity of the dorsal valve. 

 Fig. 1 & 2 a, 6, c. Dorsal, ventral, profile, and front views of this species. 

 Fig. 2. Enlargement of the beak of the dorsal valve, showing the foramen and false area. 



Fig. 3 & 4. Specimens with a more extended hinge line, and which present some slight 

 differences in the striae. 



