THE LOWER SILURIAN ROCKS. 645 



Opisthoptera extenuata, n. sp. 



Fig. 2, a-d, Opisthoptera extenuata, n. sp., upper beds of the Cincinnati group 

 Warren county, Ohio : a, the right valve described, of the natural size ; b, anterior 

 view in outline; c, cardinal view; and d, oblique cardinal view showing the flattened 

 anterior side, e, Opisthoptera a/ternata, n. sp„ upper beds of the Cincinnati group, 

 Waynesville, Ohio. Diagrammatic sketch, x 2, of the original of figure 9 on 

 plate 49. The specimen is very young and therefore has simple costse only except 

 along the front. With age their number is much increased by interpolation and 

 bifurcation, producing an appearance much as in O. fissicosta. 



Shell small, triangular, the height and length respectively as two is 

 to three; anterior side thick, flattened, with a nearly straight and rather 

 strongly oblique outline, forming an angle of about 57° with the hinge 

 line; ventral margin narrowly rounded, its center almost directly beneath 

 the middle of the hinge; posterior margin strongly insinuated a little 

 above the middle, sloping backward very gently in the lower part and 

 abruptly prolonged in the wing; hinge long, though seeming not to 

 extend quite to the sharply rounded extremity of the wing. Beaks mod- 

 erately prominent, strongly incurved. Ligamental area narrow. Beneath 

 the wing the surface of the valves is distinctly depressed. Surface of the 

 specimen described marked with about thirty small costse at the margin, 

 those on the wing larger than elsewhere. An approximately uniform 

 size is maintained by bifurcation. Greatest length 16.5 mm.; height 11 

 mm.; from beak to center of base 13.5 mm.; thickness of one valve 4.5 

 mm. 



This species seems to occupy an intermediate position between O. 

 casei and O. obliqua. For the former the anterior side is too oblique, too 

 flat, and too straight, while the posterior margin also has a much deeper 

 sinus. With the latter it agrees in the anterior and basal parts, but 

 differs very decidedly in the prolonged wing and deeply sinuate posterior 

 margin. 



Formation and locality : Upper beds of the Cincinnati group, War- 

 ren county, Ohio. The horizon is probably one hundred feet beneath 

 that occupied by 0. casei. 



