LOWER HELDERBERG ROCKS 
945 
PLATE Cl. 
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 f7, e. Dorsal and ventral views of a cast of this species. 
Fig. ‘If. 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 XXXYIII. Fig. 1-7. 
Shell ovate or suborbicular. Yentral 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 striae, 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 Terebratula duboisi of De 
Verneuil (Geologie de la Russie, PL 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. 
