PLATE LV. 



PAGE. 

 Figs, land 2 ORTHOCERAS NICOLLETI, sp. nov 784 



1 View of the portion of the conch of the original specimen; showing the strong, 



distant annulations and the obliquity of the septa, x 1 1-6. 



2 Outline of a septum; showing the position of the sipho. 



From the Trenton limestone at Belle Creek, Minn. 



Figs. 3 and 5 ORTHOCERAS OLORUS Hall 788 



3 A portion of an internal cast referred to this species. 



5 Outline of septum, showing position of sipho. 



From the Trenton limestone, Minneapolis, Minn. 



Figs. 4 and 6 ORTIIOCERAS TENUISTRIATUM Hall 788 



4 A portion of a conch protruding from an internal cast of a Cameroceras-sipho, and 



retaining the surface ornament, x 1 1-6. 



6 An enlargement of the surface; showing the fine concentric lines crossed by longi- 



tudinal bands which appear to be traces of color-streaks. Cannon Falls, Minn. 



Fig. 7 ORTHOCERAS SOCIALE Hall 789 



An internal cast apparently entire at the aperture; showing the form of the shell, 

 length of the body-chamber and the distances between the septa. 

 From the Maquoketa or Hudson River shales at Graf, Iowa. 



Figs. Sand 9 ORTHOCERAS LESUEURI, sp. nov 785 



(See also plate LIII.) 



8 An Internal cast with \ery regular annulations. Trenton limestone, Dixon, 111. 



9 An internal cast having the annulationc slightly undulating and the septa lying 



regularly in the intervening furrows. Trenton limestone, Cannon Falls, Minn. 



Fig. 10 ORTHOCERAS HELTRAMI, sp. nov 789 



The original specimen, natural size; showing the body-chamber and thirteen septa. 

 Galena limestone, Wykoff, Minn. 



