Contributions to Paloeontology. 



103 



This species is intermediate, in form, between L. antiqua and L. 

 prima of the Potsdam sandstone in New-York, but is distinctly 

 different from either of them. It occurs in the same association 

 with Dikelocephulus minnesotensis at Lagrange mountain in Min- 

 nesota, and in the same horizon and similar association near Mazo- 

 mania, Wisconsin. 



LINGrULA AURORA. 



PLATE I. FIGS. 4 & 5. 

 Lingula aurora: Hall, Annual Geological Report of Wisconsin, 1860-61. 



Shell broadly ovate : beak obtuse, appressed, a little more 

 attenuate in the ventral valve ; sides abruptly diverging 

 from the beak, including between them an angle of about 

 85°. Cardinal 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 strise ; and on the middle 

 and front of the exfoliated specimens, radiating striae 

 are more or less distinctly visible. Besides these, there 

 is a transverse, sharply undulating, or zigzag set of 

 lamellose stria? crossing the others, giving a finely check- 

 ered 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 

 Hamilton group. 1 It resembles the L. ampla of Owen; only in 

 general form and size. 



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

 the restrictions 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. 



1 Vol. iv, Palaeontology of New York, and page 21 of the Report. 



