CONTRIBUTIONS TO PALAEONTOLOGY. 127 



slope sometimes nearly straight for less than a third of the 

 length, beyond which it is regularly curved : base broadly 

 rounded, nearly flat or but little elevated. 

 Surface marked by concentric striae; and on the middle and 

 front of the exfoliated specimens, radiating stride are more or 

 less distinctly visible. Besides these, there is a transverse, 

 sharply undulating, or zigzag set of lamellose striae crossing 

 the others, giving a finely checkered or subpunctate character. 

 In partially exfoliated shells, or in the impressions in sandstone 

 where the shell is mainly dissolved, the surface has a punctate 

 aspect, and, except by a lens, is not distinguishable from such 

 texture. 



The surface characters are similar to those of L. punctata of the Hamil- 

 ton group.* It resembles the L. ampta of Owen only in general form and 

 size. 



At the time of describing L. aurora, I had not fully considered the re- 

 strictions of character and position of the L. ampla of Owen ; but on a 

 review of all the facts, I am satisfied that the latter name must apply to the 

 Trempaleau species, while the name L. aurora will be applied to the one 

 from the higher beds. 



It occurs in association with Dikelocephalus minnesotensis, near Mazo- 

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



LINGULA AURORA, var. 



PLATE VI. 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 concentric and 

 radiating strise, giving sometimes a cancellated or granulose 

 structure. The transverse lamellose striae are coarser, and not 

 so abruptly undulating as in the specimen from the gray sand- 

 stone below. 



This form occurs in considerable numbers in some red and variegated 

 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 Lingul.^ which I have 

 observed in the higher beds of the formation, with the exception 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 species 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 



VoL iv, Palaeontology of New-York, and page 21 of this Report. 



