396 THE PALEONTOLOGY OF MINNESOTA. 
[Strophomena fluctuosa. 
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 triangular; 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 striz, 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. fiuctuosa, has the convexity of its valves reversed and is, 
therefore, a species of Rafinesquina. . 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., pl. rx, figs. 10, 12-14). S. fluctuosa thus becomes a well marked 
