330 Lower Silurian Fauna of Minnesota. 



LEPTAENA RECEDENS n. Sp. 



Plate IV, figures 29, 30, 31 and 32. 



Compare L. transversalis Sowerby. 



Length 10 mm., about one-half the breadth ; rather thick. Hinge line 

 sometimes greater than represented in the figures. Surface marked by 

 fine radiae with a few coarser ones. No concentric lines on uninjured 

 shells. Ventral valve usually with a broad sinus. This form is larger 

 than L. praecosis n. sp. and is more convex anteriorly while the latter is 

 most strongly convex posteriorly. The dorsal valve is less convex than 

 that of L. transversalis Sowerby. Common in the limestone of the Cin- 

 cinnati group in Fillmore county, near Spring Valley. 



LEPTAENA SAXEA n. Sp. 



Plate IV, figures 33, 34 and 3>^. 



Length and breadth 15 and 22 mm. respectively; cardinal angles about 

 85 degrees. Ventral valve moderately convex; dorsal valve slightly con- 

 cave. Surface marked by two heavy concentric lines, besides the usual 

 radiating lines. Visceral area on the dorsal valve slightly depressed and 

 divided by four high radiating processes, the two central ones being some- 

 what the larger. Interior of the ventral valve like that of L. recedens. 



From the limestone of the Cincinnati group, near the top, town of 

 Bristol, Fillmore county, Minn. 



ORTHIS CORPULENTA n. Sp. 



Plate V, figures 8, 9 and 10. 



Compare O. testudinaria Dalman vars. Meeki and Multisecta. 



Shell circular, diameter 20 mm; hinge line equal to two-thirds the 

 greatest width. Ventral valve strongly convex, and with a sharp incurved 

 beak; surface near the beak of either valve is marked by about fifteen 

 radiating plications which bifurcate or implicate about three times before 

 reaching the mesial line. Concentric growth lines several or many. Dor- 

 sal valve convex with a broad indefinite sinus. Rarely both valves are 

 equally convex, the shell globular and the hinge area concealed. Immature 

 specimens have the general outline of O. multisecta but are less plicated, 

 and have a narrower hinge line. O. meeki is distinguished from O. corpu- 

 lenta by its less rotund form and larger, angular sinus and median ridge. 

 This is the largest form like O. testudinaria Dalman, yet seen ; specimens 

 are often larger than the ones figured. 



From the shales and limestone of the Cincinnati group, Fillmore 

 county, Minn. Specimens of the same species from Kentucky have been 

 seen. 



ORTHIS MACRIOR n. Sp. 



Plate V, figures 5, 6 and 7. 



Compare O. emacerata Hall. 



Shell semicircular, hinge line 20 mm. or less; length 15 mm. or less. 

 Ventral valve convex between the hinge line and mesial line, most strong- 

 ly so along the median ridge ; slope from this median ridge to the lateral 

 margins plane or slightly convex; beak small, extended. 



Dorsal valve plane but for a somewhat acute sinus which extends 

 and expands from the beak (of the dorsal valve) to the mesial line. The 



