178 PALAEONTOLOGY OP NEW-YORK. 
scarcely incurved. Hinge line straight, nearly equalling the greatest 
width of the shell. Area linear, plane. Foramen apparently closed. 
Surface marked by coarse radiating strios, which frequently bifurcate and 
increase by implantation : several of those on the mesial elevation of 
the ventral valve appear to coalesce along the Centre, before reaching 
the beak. In well-preserved specimens, strong concentric strife are 
visible in the depressions between the radiating strhe. Shell marked 
by a few strong concentric undulations of growth. 
The casts of the interior show the cavities made by the diverging teeth and the 
strong muscular imprints of the cavity beneath the beak, which, when well preserved, 
is cancellated by radiating and concentric lines. Vascular impressions radiating and 
ramifying from the muscular cavity, and diverging over the entire surface of the 
cast. Dorsal valve showing the cavities of the small cardinal process and the 
brachial lamellse, the double imprints of the adductor muscles, and the ramifying 
vascular impressions. 
This species is closely related to the O.fasciata of the Niagara group; but it is a 
larger shell, less angular at the extremities, and more deeply impressed along the 
centre of the valve. 
This species may be compared with 0. desiderata and 0. macrostoma of Barrande, 
which are apparently opposite valves of a similar but smaller species. 
This shell, in many of its features, holds an intermediate position to Ort/iis and 
Strophomena as defined by Davidson. 
Fig. 2 a, b, c. The ventral valves of several specimens which are more or less exfoliated, so 
that the surface characters are not fully preserved. 
Fig. 2 d, e. Casts of the ventral valve, showing the muscular and vascular impressions. 
Fig. 2 f. Profile of a dorsal valve from which the shell is partially exfoliated. 
Fig. 2 g, h. Two specimens of the dorsal valve, showing muscular and vascular impressions. 
Fig. 2 i. Enlargement of the muscular and vascular impressions, cavities of cardinal and 
brachial processes, etc. of part of a dorsal valve. 
Fig. 2 k. A farther enlargement of one side of the cast of a dorsal valve. 
Fig. 2 l. Enlargement of the striae, showing in the depressions some fine concentric striae. 
Geological position and locality. In the slialy limestone of the Lower ILelderberg 
group : Helderberg mountains and Hudson. 
V 
