98 



Contributions to Palmontology. 



occur is between four and five hundred feet. This interval 

 may be represented by a greater or less thickness in other 

 parts of the group. It may have been accumulated in a 

 comparatively short time, or it may represent a long 

 period ; for although some parts of the series appear to 

 have been rapidly accumulated, there are lines of demark- 

 ation indicating a change in the nature of the sediment, and 

 other evidences of beach and shore lines with fucodal 

 remains, together with mud-cracks and other marks of de- 

 siccation, so that we cannot be sure that the accumulation 

 was equal and constant. It . may, therefore, represent an 

 interval, which, in some other part of the country, will be 

 found occupied by a much greater thickness ; and these 

 successive trilobite beds may be representatives of that 

 which, in other localities, will be found characterized by 

 a more considerable and distinct fauna. I make this 

 suggestion, from the facts which have fallen under my 

 observation during investigations much too limited in time 

 and area to afford such satisfactory results as I could 

 desire. 



It is an interesting and significant fact that the two most 

 characteristic forms of Dikelocephalus commenced their 

 existence near the close of the Potsdam period proper, 

 passing into the alternating lower beds of the Magnesian 

 limestone; while but one other species of Trilobite has 

 been found in the same association, so far as my investiga- 

 tions have extended. 



It is, perhaps, premature to generalize from these few 

 facts ; but according to our present knowledge of the 

 Upper Mississippi valley, we are compelled to regard the 

 typical forms of Dikelocephalus as characteristic of the 

 close of the Potsdam period. 



In addition to the Trilobites, we have a single species 

 of Agnostus, and a new and remarkable Crustacean ; the 

 latter from the higher beds of the sandstone, or within 

 about fifty feet of the Lower Magnesian limestone. 



