LOWER HELDERBERG ROCKS. 193 



This species is one of a small group including S. pecten and S. subplana, which 

 possess characters assimilating them with Orthis. In the present example, the 

 strongly marked vascular impressions, and the bifid and lobed cardinal process 

 filling the lower part of the foramen, are characteristic of Orthis, while the closed 

 foramen and linear area are characters of Strophomena. 



Fig. 1 a. Ventral valve of an individual of medium size. 



Fig. 1 b. Dorsal valve of a larger individual, showing the area of the opposite valve and 

 the closed foramen. 



Fig. 1 c. Ventral valve of the same. 



Fig. 1 d. Profile view of the same. 



Fig. 1 e. Ventral valve of a large individual. 



Fig. 1 f. View of area of fig. 1 e. 



Fig. 1 g. Cardinal view of an entire individual, showing the area of the ventral valve, and 

 the convexity of the dorsal valve. 



Fig. 1 h, i, k, I. Interior of the ventral valves of several individuals of different size, and 

 showing some variety in form of the vascular impressions. 



Fig. 1 m. A cast of the ventral valve. 



Fig. 1 n, o. Interior of dorsal valves, showing the cardinal processes, with some difference 

 in the strength of the muscular impressions and proportional length of the hinge 

 line. 



Fig. 1 p, r. Cardinal view of ventral valve, showing the cardinal process, and the same 



enlarged. 

 Fig. 2 a, 6, c. Casts of the interior of the dorsal valves of several individuals. 



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

 group : Helderberg mountains; Schoharie, Carlisle, Hudson and Catskill. 



Strophomena radiata. 



Plate XXI. Fio. 8 & 9 ; .and Plate XVIIL Fig. 3 b,.c, d, e. 

 Strophomena radiata : Vanuxem, Report Third District of New- York, pa. 122, f. 6. 



Shell nearly semicircular, about three-fourths as long as wide. Ventral 

 valve flat or subconcave, except near the umbo, where it is depressed 

 convex : beak rising a little above the line of the cardinal margin. 

 Dorsal valve flat or slightly concave near the beak and towards the 

 extremities, very gibbous in the middle and towards the front : beak 

 not elevated above the cardinal margin. Hinge line equalling or greater 

 than the greatest width of the shell, apparently not crenulated. 



Surface marked by distinct radiating striae, which increase by implanta- 

 r PAL^EONTOLoar III.] 25 



