PLATE LVII. 



PAGE. 

 Fig. 1 TRIPTOCERAS PLANOCONVEXUM Hall 791 



(See also plate L.VI.) 



A partial internal cast of a large shell Galena limestone, Hader, Minn. 

 Figs. 2 to 4 TRIPTOOERAS PLANODORSATUM Whitfleld 792 



(See also plate i.vi. i 



Three views of an internal cast of the body-chamber. 

 Trenton horizon, Minneapolis. 



Fig. 5 GONIOCERAS ANCEPS Hall 794 



A partial internal cast; showing the form and depth of the air-chambers, the size 

 of the sipho and the filling of the branches of its internal canal. Trenton 

 horizon, Minneapolis. 



Fig. 6 GONIOCERAS OCCIDENTALS Hall 795 



A weathered specimen, showing the double curvature of the septa, the size of the 

 lateral flange and of the slpho. Trenton limestone, Dixon, Illinois. 



Figs. 7 to 10 CLINOCERAS MUMI^EFORME Whitfleld 797 



7 An internal cast of the body-chamber. 



8 A nearly entire individual retaining the shell. 



9 An enlargement of its surface. From the Trenton horizon, locality uncertain. 



10 An internal cast of a portion of the conch; showing the depth of the air- 

 chambers, the position and form of the sipho. x 1 1-6. 



Fig. 11 POTERIOCERAS APERTOM Whiteaves 796 



A view of the best specimen observed, retaining most of the body-chamber and 

 eleven air-chambers. Galena limestone, St. Paul, Minn. 



