LOWER HELDERBERG ROCKS. 
173 
This species bears some resemblance to 0. elegantula of the Niagara group; but 
it is more finely and beautifully striated, the dorsal valve is more gibbous, and 
distinctly marked by a rounded depression down the centre; the hinge line is more 
extended; the area is higher; the ventral valve is more gibbous, and the beak less 
arched. 
In some points, the form of this species resembles O. parva , a Lower Silurian 
species; but the dorsal valve is usually more gibbous, while the stride are finer, 
more equal, and less disposed in fascicles. 
Pig. 1 a, b, c, d. Ventral, dorsal, profile, and cardinal views of a small specimen. 
Fig. 2 a. A larger individual. 
Fig. 2 b. Enlargement of the area and foramen. 
Fig. 3. Enlargement of strise. 
Geological position and locality. In the shaly limestone of the Lower Helderberg 
group : Cumberland, Maryland. 
A-c/Vv * , H cJTC , / U 
1 Ortliis insignis (n. s ). 
Plate XIII. Fig. 13 - 15. 
Shell pyramidal, minute. Dorsal valve semicircular, nearly flat, with a 
well-marked sinus down the middle. Ventral valve extremely elevated, 
pointed, straight or incurved. Hinge line greater than the width of the 
shell. Area large, extending to the salient hinge extremities : foramen 
large. 
Surface coarsely striated : strise rarely bifurcating. 
This shell bears some resemblance to O. pyramidalis of the Niagara group; but 
it is more angular in form, the dorsal valve is conspicuously depressed in the centre, 
and the beak of the ventral valve is more produced. 
Fig. 13. Dorsal view, natural size. 
Fig. 14 & 15. Dorsal valve and area magnified. 
Geological position and locality. In the shaly limestone of the Lower Helderberg 
group : Helderberg mountains. 
