LOWER HELDERBERG ROCKS. 17» 



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 mai'ked 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 0. parva, a Lower Silurian 

 species; but the dorsal valve is usually more gibbous, while the Striae are finer, 

 more equal, and less disposed in fascicles. 



Fig. 1 a, b, c, d. Ventral, dorsal, profile, and cardinal views of a small specimen. 



Fig. 2 a. A larger individual. 



Fig. 2 6. Enlargement of the area and foramen. 



Fig. 3. Enlargement of striae. 



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

 group : Cumberland, Maryland. 



Orthis insignis (n.s.). 



Plate XIII. Fig. 13 - 15. 



Shell pyiamidal, 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 : striae rarely bifurcating. 



This shell bears some resemblance to 0. 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 & 1.5. Dorsal valve and area magnified. 



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

 group : Helderberg mountains. 



