THE LOWER SILURIAN ROCKS. 659 



Associated with this species and A. cincinnatiensis I found a single 

 imperfect left valve, agreeing very closely with A. modioliformis ; Meek 

 andWorthen, sp. 



Formation and locality: L,ower 75 feet of the Cincinnati group, at 

 Covington, Kentucky, where it has been found only in the banks of the 

 Ohio river. 



Genus COLPOMYA, Ulrich. 



(Final Rep. Geol. Sur. Minn., Vol. Ill, p. 522. (In press.) 



Colpomya constricta, n. sp. 



Plate 52, Figs. 17-19. 



Shell somewhat elongate, strongly convex on the umbonal ridge, 25 

 mm. to 35 mm. in length, 14 mm. to 18 mm. in height at the beaks and at 

 the posterior extremity of the hinge; thickness about three-fourths of 

 the height. Outline subrhomboidal, with the hinge line straight, and 

 extending about two-thirds of the entire length; anterior margin curv- 

 ing neatly backward from the sharply rounded or subangular extremity 

 of the hinge; ventralmargin gentle sinnate ; posterior margin very oblique, 

 produced and strongly rounded below, but littte curved in the cen- 

 tral and upper parts; post-cardinal angle about 135°. Beaks strongly 

 incurved, situated one-third or more of the length of the hinge line from 

 the anterior extremity; umbones large, prominent, distinctly constricted 

 by the small end of the strongly developed, oblique mesial sulcus; pos- 

 terior umbonal ridge very prominent, not angular but sharply rounded, 

 distinguishable from the beak to the post-basal angle; anterior umbonal 

 ridge consisting of a broad swelling in front of the mesial depression; 

 cardinal slope abrupt, concave. Surface marked by somewhat unequal 

 though on the whole strong concentric lines of growth. 



Hinge plate straight and very thin posterior to the beaks, twice as 

 strong in front of them; beneath the beaks of the right valve a tooth 

 like prominence that fits into a corresponding depression beneath the 

 beak of the left valve; in front of and beneath this pit in the left valve a 

 strong process projects obliquely downward, backward and toward the 

 opposite valve and is partly received in a socket that defines the anterior 

 side of the tooth in the right valve, while its lower end curves under that 

 tooth. Anterior muscular scar subovate, not very large, distinct, bounded 

 upon the inner side <by a thin ridge running down from the hinge. 

 Immediately behind the top of this ridge a small pedal muscle scar is 

 excavated out of the under side of the hinge plate. Posterior scar and 

 pallial line not satisfactorily shown in the specimens at hand. 



Two other species of Colpomya are associated with the present form, 

 but as they are much smaller and obviously different in their outlines, 

 there is little likelihood of confusion between them. 



