404 THE PALEONTOLOGY OF MINNESOTA. 
[Variety Inquassa. 
Original description: " Shell deltoid, with numerous radiating strise 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; strise 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 Rafinesguina alternata, 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 B. 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 irilobata, 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. trilohata 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. Whitfield gives it 
as occurring in the Trenton, Galena and ?Hudson River group of Wisconsin. In the Trenton of New 
York and Canada. Davidsont mentions it as occurring in the Caradoc or Bala period in England, Scotland 
and Ireland, also "atPaggart, 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. Scofleld 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 Rep. N. Y. Geological Survey, p. 115 (undefined) 
1838-41. 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, fig. 3. 
"Pal. N. Y., vol, i, p. 107. 
+ Monograph of British Silurian Bracliiopoda, p. 292. 
