508 THE PALEONTOLOGY OF MINNESOTA. 
(Modiolopsis mytiloides. 
MoproLtopsis mytiLtorpEs Hall. 
PLATE XXXVI, FIG. 8. 
Modivlopsis mytiloides HALL, 1847. 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 in 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 locality.—Trenton limestone, Middleville, New York; middle Galena, Goodhue and 
Fillmore counties, Minnesota, and Oshkosh, Wisconsin. According to Billings, in the Trenton and Black 
River groups of Canada. J 
Mus. Reg. No. 8361. 
MopioLopsis CHATFIELDENSIS, %. Sp. 
PLATE XXXVI, FIGS. 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 3.38 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 M. subparallela Ulrich, also occurring in that higher series of 
strata at Covington, Kentucky. Compared with Minnesota species, it is perhaps 
