404 THE PALEONTOLOGY OF MINNESOTA. 



[Variety Inijuassa. 



Original description: " Shell deltoid, with numerous radiating striae and concentric 

 rugose undulations, obsolete on the inferior half of the valves; inferior valve slightly 

 convex above, gibbose, abruptly rounded and flattened at the base; striae small and 

 crowded; one or two lines in the middle of the valve larger and more prominent 

 than the others; angles of the cardinal line slightly prominent. Length, one inch. 

 Locality, Trenton Falls." 



This species is closely related to Rafinesquina alternate*, and differs from it both 

 in its greater convexity and in the corrugations of the central disc. The latter 

 feature is never very well developed in Minnesota specimens, while the convexity 

 may be very great as in camura, with all variations to those nearly flat. These 

 depressed convex specimens, especially when the concentric corrugations are obso- 

 lete, approach R. alternata so closely that it is difficult or impossible to separate 

 them. Such forms are, however, rare. This same difficulty is also met with in 

 New York specimens. Prof. Hall writes,* " it is certainly often very difficult to draw 

 the line of distinction between this species [R. deltoidea] and L. alternata, and more 

 particularly so'between this and L. camerata." 



R. deltoidea is associated with Strophomena trilobata, a species with about the 

 same curvature and corrugations of the central disc. The latter can be readily 

 distinguished by the reversed convexity of the valves, the upper, or strongly rounded 

 valve being the dorsal, while in R. deltoidea this is the ventral valve. The nasute 

 anterior portion of the shell and the small, flat, or even slightly concave, central 

 disc will also assist in separating S. trilobata from R. deltoidea. 



Formation and locality. From the top of the Trenton shales at St. Paul and Cannon Falls, Minne- 

 sota, examples have been found which probably belong to this species. Near the middle of the Galena 

 formation at Mantorville, it occurs commonly as casts and retains more or less of the shell at several 

 localities in Goodhue county, and at Weisebach's dam near Spring Valley; close to the top of the forma- 

 tion near Hamilton, and in the lower portion of the Hudson River group at Granger, Minnesota. In the 

 Galena at Oshkosh, Wisconsin, and near the top of the hills at Dubuque, Iowa. Prof. Whitfleld gives it 

 as occurring in the Trenton, Galena and ?Hudson River group of Wisconsin. In the Trenton of New 

 York and Canada. Davidsonf mentions it as occurring in the Caradoc or Bala period in England, Scotland 

 and Ireland, also "at Paggart, in Esthonia, and at Reval; in Norway and elsewhere." It is believed by 

 the writers, however, that a direct comparison of the British examples referred to R. deltoidea will prove 

 them to be different from American specimens in their muscular markings and crural plates. 



Collectors. M. W. Harrington, W. H. Scofteld and the writers. 

 Mus. Reg. Nos. 174, 182, 204, 261, 387, 389, 394, 3395, 8157-8164. 



RAFINESQUINA ALTERNATA (Conrad Ms.) Emmons. 



PLATE XXXI, FIGS. 32-34. 



1838. Leptcena alternata CONKAD. Second Annual Eep. N. Y. Geological Survey, p. 115 (undefined) 

 1838^1. Strophomena alternata CONRAD. Ibidem, Third Report, p. 63; Fourth Report, p. 201; 

 , Fifth Report, p. 37 (undefined). 



1842. Strophomena alternata EMMONS. Geology of New York; Report Second District, p. 395, flg. 3. 



* Pal. N. Y., vol, i, p. 107. 



tMonoKraph of British Siluriau BrachiopoUa, p. 292. 



