192 Contributions to Palozontology. 



i 



overlying the fossiliferous sandstone, either with or with- 

 out the intervention of the Lower Magnesian limestone. 



I have appended these few facts and arguments, with a 

 view of presenting, in connextion with this notice, some 

 of the points of interest yet remaining undetermined in 

 regard to the older deposits of the West, and the difficul- 

 ties in the way of determining their satisfactory parallel- 

 ism with those of the East, considering simply the sequence 

 of formations as originally presented in the State of New 

 York. 



ADDENDUM. 



ON SOME FOSSILS OF THE LOWER BEDS OF THE POTSDAM SANDSTONE 

 OF THE UPPER MISSISSIPPI VALLEY. 



Since the completion of the printing and engraving of 

 the plates relating to the fossils of the sandstone of the 

 Upper Mississippi valley, a re-examination of some of the 

 specimens from the lower beds near Trempaleau, by Mr. 

 Whitfield, has brought to light several forms, quite new 

 and distinct from any before noticed. One of them, a 

 Crustacean somewhat resembling the pygidium of a Trilo- 

 bite, has a very narrow axis aud large swollen lateral 

 lobes. It differs from the pygidium of a trilobite, in the 

 extension of the narrow axis quite to the margin of the 

 shield. On the straight side, the articulating face (if such 

 it be) differs essentially from the corresponding parts of a 

 trilobite. In these respects, also, it differs from Agnostus. 



From its remarkable characters, it seems necessary to 

 designate it as an undescribed generic form; and I propose 

 the name Pemphigaspis. 



