396 THE PALEONTOLOGY OF MINNESOTA. 



LStrophomena fluctuosu. 



Original description: "Triangular or semioval, usually widest at the hinge-line 

 and more or less narrowly rounded, pointed, trilobed or nasute in front. 



" Dorsal valve convex, the visceral disc being in general equal to one-third the 

 superfices of the whole valve, nearly flat, the remainder abruptly curved down all 

 around so that the lower half of the length of the shell is sometimes at right angles 

 with the upper half. The cardinal angles more or less compressed and often a little 

 reflected, usually forming angular or narrowly rounded ears. Ventral valve concave, 

 the curvature corresponding to that of the dorsal valve. 



"Area of dorsal valve lying in the plane of the lateral margin, about one-third 

 of a line high. Area of ventral valve forming a right angle with the marginal plane, 

 in large specimens one line or a little more in hight at the beak, and gradually 

 decreasing towards the extremities of the hinge-line. 



"Foramen of ventral valve triangulaj; the width at the base somewhat exceed- 

 ing the hight, completely closed by a convex deltidium, the basal margin of which 

 is rendered a little concave by the convex margin of the similar deltidium [chili- 

 dium] which closes the foramen of the dorsal valve. 



" Surface with a set of fine, rounded, elevated, radiating striae, distant from each 

 other usually about half a line, sometimes a little less, and occasionally one line. 

 Between each two of these there are from two to ten much finer striae; the whole 

 crossed by fine, crowded, concentric lines. In most of the specimens the whole 

 of the upper half of the shell is covered with short undulating wrinkles, which 

 sometimes have a concentric arrangement and often form concentric rows con- 

 verging from the hinge-line towards the center of the shell, crossing each other. 

 The specimens from the Trenton limestone are usually without these undula- 

 tions, [probably S. winchelli], but in those from the Hudson River group this 

 character is prominently exhibited." Interior of both valves very much as in S. 

 nutans (James) Meek, or S. rugosa Blainville. 



This species has been confounded with Rafinesquina deltoidea Conrad, sp,, as 

 figured by Prof. Hall (Pal. New York, vol. i). The types now in the American 

 Museum of Natural History, in New York city, have been carefully examined by 

 Prof. Hall, Mr. Clarke and one of the writers. After considerable difficulty, owing 

 to the thinness of the shells and the limestone matrix, it was proved that R. deltoidea, 

 when compared with S. fluctuosa, has the convexity of its valves reversed and is, 

 therefore, a species of Rafinesquina. R. deltoidea must therefore be restricted to the 

 specimens figured by Hall in 1847. Trenton shells from Canada and Wisconsin, 

 devoid of the corrugated surface, and usually referred to this species, are removed 

 from S. fluctuosa and R. deltoidea and used as the type of a new species, S. winchelli 

 Hall (op. cit., pi. ix, figs. 10, 12-14). S. fluctuosa thus becomes a well marked 



