PLATE LVI. 



PAGE. 

 Figs. 1 and 2 TRIPTOCERAS LAMBI Whiteaves 987 



1 View of a large specimen. 



2 One of the septa; showing the form of the shell in cross-section and the position 



of the sipho. From the Galena limestone, Stewartville, Minn. 



Fig. 3 TRIPTOCEBAS PLANOCONVEXUM Hall 791 



(See also plate i.vn < 



View of an internal cast. 

 Trenton limestone, Cannon Falls, Minn. 



Fig. 4 TKIPTOCERAS PLANODORSATUM Whitfleld 792 



(See also plate Lvn.) 



View of the flatter side of an internal cast; showing the curvature of the septa. 

 Trenton limestone, Minneapolis. 



Figs. 5 to 7 TKIPTOCERAS OWEXI, sp. nov 792 



Three views of the typical specimen, showing the contour of its surfaces. 

 Trenton limestone, Cannon Falls. 



