LOWER HELDERBERG ROCKS. 
193 
This species is one of a small group including S. pecten and S. subplan a , which 
possess characters assimilating them with Ort/iis. In the present example, the 
strongly marked vascular impressions, and the bifid and lobed cardinal process 
filling the low r er part of the foramen, are characteristic of Or this, while the closed 
foramen and linear area are characters of Strophomena. 
Fig. 1 a. 
Fig. 1 b. 
Fig. 1 c. 
Fig. 1 d. 
Fig. 1 e. 
Fig- If. 
Fig. 1 g. 
Fig. 1 h, 
Fig. 1 m. 
Fig. 1 n, 
Fig- 1 P , 
Fig. 2 a, 
Ventral valve of an individual of medium size. 
Dorsal valve of a larger individual, showing the area of the opposite valve and 
the closed foramen. 
Ventral valve of the same. 
Profile view of the same. 
Ventral valve of a large individual. 
View of area of fig. 1 e. 
Cardinal view of an entire individual, showing the area of the ventral valve, and 
the convexity of the dorsal valve. 
i, k, l. Interior of the ventral valves of several individuals of different size, and 
showing some variety in form of the vascular impressions. 
A cast of the ventral valve. 
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. 
r. Cardinal view of ventral valve, showing the cardinal process, and the same 
enlarged. 
b, 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. 
Strophoiiieiia radiata, 
Plate XXI. Fig. 8 & 9; and Plate XVIII. Fig. 3 b, c, d, c. 
Strophomena radiata : Vanuxeh, 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- 
[ Paleontology III.] 25 
