522 THE PALEONTOLOGY OF MINNESOTA. 



[Modlolodon (?)gibbus. 



the cardinal region compressed and rounding, except in the youngest stages, gradu- 

 ally into the posterior margen. Beaks small, rather prominent, scarcely incurved 

 in the shell and not at all in the casts. Surface of casts almost uniformly convex. 

 Pallial line distinct along the anterior and ventral margins, not traced posteriorly; 

 nor has the posterior muscular scar been observed. Hinge with two interlocking 

 cardinal teeth in the left valve, and corresponding sockets and teeth in the right. 



This shell is wider, more erect and more uniformly convex than M. oviformis, 

 the type of the genus. The erectness of the beaks is a very unusual feature among 

 the Modiolopsidce and should render good service in the identification of the species. 



Formation and locality. Middle Galena, Kenyon, Goodhue county, Minnesota, and Decorah, Iowa; 

 also in the Trenton near Danville, Kentucky. Rare. 



Mus. Reg. No. 8363. 



MOBIOLODON (?) GIBBU8, . Sp. 



PLATE XXXV. FIGS. 28 and 29. 



Shell small, obliquely ovate, the anterior end very small, separated as a bicarin- 

 ated lobe from the body of the shell by a distinct sulcus extending vertically across 

 the valves from the anterior side of the beaks. Behind this sulcus the valves are 

 gibbous, especially in the umbonal region and anterior to the center; posterior 

 cardinal region somewhat compressed; beaks full, prominent, incurved. Surface 

 with simple concentric lines of growth, rather stronger in the sulcus than else- 

 where. Hinge very thin immediately under and behind the beaks. It widens some in 

 front of them, and here the left valve exhibits a small protuberance. Being a small 

 shell, and the specimen not very well preserved, the nature of this protuberance has 

 not been determined with certainty. Examined under a good lens it looks like the 

 remains of a double tooth. Muscular scars and pallial line not observed. 



This species cannot be confounded with any Lower Silurian bivalve known to 

 me. The small size and peculiar character of the anterior end, and the unusual 

 gibbosity of the shell, render its systematic position doubtful. I place it with 

 Modiolodon chiefly because the outline is much like that of -M. patulus, but I 

 suspect strongly that it belongs to an undescribed genus. 



Formation and locality. Upper third of the Trenton shales near Cannon Falls, Minnesota. 



Genus COLPOMYA, n. gen. 



Shell subelongate, oblique^ inequilateral, subrhomboidal or ovate in outline, 

 widest posteriorly or with the ventral and dorsal margins nearly parallel. Mesial 



