PLATE CXXV. 
Nautilus (Discites) ammonis. 
Page 38. 
Lateral view of the internal mould, showing the expanded apei’ture, 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. 
