400 THE PALEONTOLOGY OF MINNESOTA. 
[Rafinesquina 
immature examples of the latter of the same size as adult individvals of S. emaciata 
be examined, it will be seen that a greater number of less conspicuous striae are 
present, that the shell near the anterior margin is decidedly more convex, and that 
the umbo is not depressed as in S. emaciata, the last feature being more or less 
strongly developed in all the concavo-convex Strophomenas. The subcarination of 
the ventral valve of S. emaciata is also present in S. trentonensis, but in the former 
the shell is evenly convex from the cardinal line to the anterior margin, while in 
the latter it is concavo-convex. With these constant differences there is no ground 
for assuming that S. emaciata is the young of S. trentonensis. In S. scofieldi it is seen 
that the hinge line is somewhat shorter, that the valves are more convex, and that 
the fold and sinus are just the reverse of those in S. emaciata. 
Formation and locality. — Several specimens have beeo collected by Mr. W. H. Scofleld near the base 
of the Galena in the Clitambonites beds south of Cannon Falls, Minnesota. 
RAFINESQUINA, Hall, 1892. 
Strophomena of most American authors. 
1892. Rafinesquina, Hall. Palfeontology of New York, vol. viii, pt. i, p. 280. 
Description: "Shells normally concavo-convex. Surface ornamented by radi- 
ating striae of alternating size, crossed and crenulated by finer concentric stria\ 
Cardinal margins without denticulations. Interior of the pedicle valve with the 
muscular area not strongly limited; consisting of two broad, flabellate. diductor 
scars enclosing an elongate, more distinctly defined adductor. The faintness of the 
limitation of this area is in marked contrast to the sharply defined muscular area in 
the corresponding valve of Leptcenn. In the brachial valve the cardinal process is 
more closely sessile than in Leptama, and there is frequently a linear callosity between 
the branches. The posterior adductor scars have the arborescent markings of Lep- 
toina rhomhoidalis, and these impressions are the only ones well defined, the anterior 
scars being narrow and rarely retained with distinctness. From the anterior margin 
of the muscular area radiates a series of irregular furrows and nodose ridges, which 
are, to some extent, of vascular origin. 
"Type: Leptcma alternata Conrad. Trenton and Hudson River groups." 
Rafinesquina had its origin in the Calciferous and died out in the Clinton group. 
