332 Lower Silurian Fauna of Minnesota. 



cations of a median ridge except near the beak. Beak extended and re- 

 curved. Area mostly on the ventral valve. 



The specimens figured were collected from the Cincinnati group, at 

 Wilmington, 111. Two specimens only have been found in Fillmore 

 county, Minn., towards the top of the limestone formation of the Cincin- 

 nati group. 



The relation between this species and O. corpulenta, described above, 

 is close; but smaller size, absence of cardinal angles, proportionately 

 larger muscle scars, acute mesial angle and radiately striated internal 

 surface are marks of the former. Compared with O. hybrida, Sowerby, 

 it is broader, less equivalve, has a less incurved beak, besides some in- 

 ternal differences. 



ORTHIS MINNESOTENSIS n. Sp. 



Plate v, figures 14, 15, 16 and 17. 



Compare O. pectinella Hall, O. subquadrata Hall, and O. iphigenia Billings. 



Shell broadly oval transversely; hinge line less than the greatest 

 width, which is about 15 to 20 mm. Dorsal valve convex, ornamented by 

 about twenty radiating plications on the umbo which bifurcate or im- 

 plicate once or twice before reaching the mesial line; one or two run out 

 on the hinge line on either side. Beak of the dorsal valve short, slightly 

 incurved. V&ntral a little convex, more strongly so near the beak, which 

 is short, but a little curved. Area of the ventral valve broad, with the 

 deltidium extending to the beak. 



This species agrees in size and general form with O. pectinella, Con- 

 rad. Immature specimens are scarcely distinguishable, since in form and 

 size the two are very like. But mature specimens dififer radically in the 

 manner of the plications. 



From the Fucoid bed upwards into the Camarella bed; very gen- 

 erally distributed in Minnesota. 



ORTHIS PETRAE n. sp. 



Plate V, figures 18, 19, 20 and 21. 



Shell subquadrate, with rounded corners. Length about "15 mm., 

 breadth 20 mm. Dorsal valve convex, marked by fifteen to twenty radiat- 

 ing plications near the beak ; of these central ones continue simple to the 

 anterior margin, while those ending on the lateral margins bifurcate 

 once or twice; broad indefinite sinus on the dorsal valve, small fold on 

 the ventral. Ventral valve slightly convex, plicated like the dorsal. De- 

 rivation a proper name. 



Found in the limestone of the Cincinnati group, at Spring Valley, and 

 elsewhere in Fillmore county, Minn. 



, PRODUCTELLA MINNEAPOLIS n. Sp. 



Plate IV, figures 11 and 12. 

 Shell small, length 13 mm.; breadth 12 mm., greatest in front of the 

 middle. Ventral valves only seen ; these are convex, symmetrically round- 

 ed, the curvature from the beak to the opposite point on the mesial line 

 being a gentle coil. Hinge less than the greatest width, and slopes back 

 from the beak on either side. No cardinal area is shown by the speci- 



