508 THE PALEONTOLOGY OF MINNESOTA. 



[Modlolopsis mytiloides. 



MOBIOLOPSIS MYTILOIDES Hall. 



PLATE XXXVI, FIG. 8. . . 



Modiulopsis mytiloides Hall, J847. Pal. New York, vol. i, p. 157. 



Three incomplete casts of the interior are referred to this species. They agree 

 very well with Hall's description and figures, except in being proportionately higher. 

 But the general appearance of his figure 4a, particularly in the abruptness of the 

 postero-basal curve, causes me to believe that the original of the figure has been 

 compressed vertically and is therefore narrower than normal. 



Compared with M. arguta and M. nana, which are closely simulated, it is found 

 to differ ia its surface markings, which are fine, with stronger wrinkles of growth, 

 the latter showing only on casts; the concentric lines are, therefore, not equal nor 

 thread-like. The outline differs in the subangular junction of the posterior and 

 cardinal margins. The mesial depression also is more pronounced and the end of 

 the casts in front of the depression more swollen, causing a slight concavity in the 

 ventral margin. 



Formation and loeality.—Tienton limestone, MldcUevlUe, New York; middle Galena, Goodhue and 

 Fillmore countiesj Minnesota, and Oshkosh, Wisconsin. According to Billings, in the Trenton and Black 

 River groups of Canada. 



Mus. Reg. No. 8361. 



MODIOLOPSIS CHATFIBLDENSIS, W. Sp. 

 PLATE XXXVI, PIGS. 9 and 10. 



Shell small, subelongate, the length a little less than twice the hight. Dorsal 

 and ventral margins nearly straight, subparallel, diverging slightly posteriorly; 

 anterior end rather long, rounded; posterior margin broadly rounded, scarcely 

 oblique, curving gradually into the hinge line. Beaks compressed, projecting little, 

 situated about one-fourth of the entire length from the anterior extremity. Valves 

 moderately convex, thickest at the middle, the umbonal ridge sharply rounded in 

 the upper half; mesial flattening distinct, very gently concave. Surface of cast 

 exhibiting rather broad and unequal concentric furrows which, on the shell itself, 

 seem to have separated sharply-elevated lines. The latter were probably restricted 

 to the cardinal and posterior slopes. Anterior adductor scar large, its inner edge 

 sharply defined and curving forward. Hinge apparently thin and edentulous. 



Length 10 mm., posterior hight 5.2 mm., anterior hight 4.5 mm., thickness 3.3 mm. 



This species is not elongate, like the M. angustata Ulrich, of the Cincinnati rocks, 

 its anterior end is shorter and the sides of the valves flatter; with a better defined 

 umbonal ridge than in 3f. subparallela Ulrich, also occurring in that higher series of 

 strata at Covington, Kentucky. Compared with Minnesota species, it is perhaps 



