178 PALEONTOLOGY OF NEW-YORK. 



scarcely incurved. Hinge line straight, nearly equalling the greatest 

 width of the shell. Area linear, plane. Foramen apparently closed. 

 Sdbface marked by coarse radiating striae, 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 striae are 

 visible in the depressions between the radiating striae. 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, wlien 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 lamellae, 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 i^ 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 O. macrostoma of Babrande, 

 which are apparently opposite valves of a similar but smaller species. 



This shell, in many of its features, holds an intermediate position to Ort/iU and 

 Strophcmena as defined by Davidson. 



Fig. 2 a, h, c. The ventral valves of several specimens wliich 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/. Profile of a dorsal valve from which the shell is partially exfoliated. 

 Fig. 2 g, A. 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 /. Enlargement of the strise, showing in the depressions some fine concentric stria:. 



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

 group : Helderberg mountains and Hudson. 



