PLATE CXXV. 



Nautilus (Discites) ammonis. 



Page 38. 

 Lateral view of the internal mould, showing the expanded aperture, the extent of the chamber of 

 habitation, the depth of the air-chambers, the direction of the septa, and the large open 

 umbilicus. 



Upper Helderberg limestone. In the drift, Ann Arbor, Michigan. 



