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 5. 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 grr and 

 for assuming that S. emaciata is the young of S. trentonensis. In S. scofieldi it i j 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 been collected by Mr. W. H. Scofleld near the base 

 of the Galena in the CUtambonites beds south of Cannon Falls, Minnesota. 



RAFINESQUINA, Hall, 1892. 



, 



Strophomena of most American authors. 



1892. Rafinesquina, HALL. Palaeontology 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 striae. 

 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 Leptcena. In the brachial valve the cardinal process is 

 more closely sessile than in Leptcena, and there is frequently a linear callosity between 

 the branches. The posterior adductor scars have the arborescent markings of Lep- 

 tcena rhomboidalis, 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: Leptcena alternata Conrad. Trenton and Hudson River groups." 

 Rafinesquina had its origin in the Calciferous and died out in the Clinton group. 



