28 Natural History Bulletin. 



Concerning the associated species at Fayette we may ob- 

 serve that Athyris vittata is present, but in greatly diminished 

 numbers; the Cyrtina has become a true C. hamiltoncnsis ; 

 the Airypa retiailaris has the same globose form as farther 

 south, but attains a very large size; S. aspera has entirely 

 disappeared, at least none are represented in all the collections 

 from that locality, although collections have been made at 

 intervals for the past twelve years. 



The most striking feature, however, of the assemblage of 

 fossils at Fayette is the remarkable development in numbers 

 and size of Terebrattila. Terebratulas are very rare fossils in 

 Muscatine and Scott counties; they are more abundant in 

 some beds near Iowa Cit}^; they become the predominant 

 species, literally crowding some of the beds, at Fayette. 



Our two large spirifers, S. ■pennata and S. parryana re- 

 spectively, mark two distinct platforms in the Hamilton strata 

 of Iowa, and each is associated with its own special assemblage 

 of fossil species. A third very distinct platform, with a 

 distinct grouping of species, is represented by the Rockford 

 shales and characterized by the presence of Spirifcra zvhit- 

 neyi and S. htmgerfordi. The discussion of this thn-d plat- 

 form must be reserved for a subsequent paper. 



DESCRIPTION OF A NEW SPECIES OF SPIRIFER FROM THE HAMILTON 

 GROUP, NEAR IOWA CITY, IOWA. 



By S. CALVIN. 



Spirifera urhana^ n. s. Shell of medium size, sub-triangular 

 in outline, slightly longer than wide, valves unequally convex, 

 hinge line in the type specimen about equal to half the great- 

 est width. 



Dorsal valve transversely elliptical, regularly convex, most 



