104 



Contributions to Palaeontology. 



It occurs in association with Dikelocephalus minnesotensis, near 

 Mazomania, Wisconsin, in the upper part of the Potsdam sandstone. 



LINGULA AURORA, var. 

 PLATE I. FIGS. 6-8. 



Shell ovate, depressed convex, one valve apparently a 

 little longer than the other : apex obtuse. 



Surface, in the casts and partial casts, marked by concen- 

 tric and radiating striae, giving sometimes a cancellated 

 or granulose structure. The transverse lamellose strise 

 are coarser, and not so abruptly undulating as in the 

 specimen from the gray sandstone below. 



This form occura in considerable numbers in some red and varie- 

 gated calcareo-magnesian layers near the top of the Sandstone series, 

 and near the base of the Lower Magnesian limestone. 



This and the preceding species are the only Lingula which I 

 have observed in the higher beds of the formation, with the excep- 

 tion of a few obscure specimens, the characters of which cannot be 

 fully determined. 



Although I have heretofore been disposed to consider that at least 

 one of the spcies of Lingula of the Potsdam of New York occurs 

 in the North-west (and the same has been recognized by Dr. Owen), 

 nevertheless, after a careful examination of the collections in my 

 possession, I am unable to satisfy myself that either L. prima or L. 

 antiqua has been found in the Mississippi valley. The species of 

 Lingula present so few salient characters, that it is not surprising 

 that cursory examinations of these fossils, as they are crowded to- 

 gether in great numbers in the beds at St. Croix falls, or when 

 scattered in the sandstone, poorly preserved and often distorted, 

 should sometimes lead to erroneous determinations, which can only 

 be corrected by longer continued examinations and comparisons. 



Dr. Owen has described, under the name Lingula pinna- 

 formis, the spatuliform Lingula from the St. Croix falls, 

 as follows : 



