PLATE LI. 



Upper tier, Streptelasma corniculum. Four specimens on left side from the 



Trenton group of Michigan. 

 Group of seven specimens on right side found in the Hudson River group ; 



the largest one on Drummond's Island, the others at Richmond, Indiana. 

 Lower tier, upper central specimen, and lowest one in the right corner, Zaphren- 



TIS UMBONATA. 



Other specimens, excepting the left outer and the third in the lower row,' are 

 forms of Zaphrentis Stockesii, found at Point Detour, in the Niagara 

 group. 



The two excepted lower specimens on the plate are not nearer determined. 



