544 THE PALEONTOLOGY OF MINNESOTA. 
(Cyrtodonta pesrsimilis. 
Formation and locality.—The original type is from the middle third of the Trenton shales at Minne- 
apolis. A small cast of the interior, belonging to the survey collection, was found in the building lime- 
stone at the same place. Casts occur also in the lower Trenton limestone at Beloit, Wisconsin and Dun- 
leith, Illinois. 
Mus. Reg. No. 5100. 
CYRTODONTA PERSIMILIS, 7. sp. 
PLATE XXXIX, FIGS. 41 and 44. 
This form, which is known only from casts of the interior, was confused with 
C. glabella until a critical comparison proved it to be not only distinct but to belong 
to another group of species. The outline is very much alike in the two species, but 
here even some constant differences are to be observed, especially in the shape of 
the margin at the posterior extremity of the hinge, where the present species is 
more angular. But the main difference lies in the fullness of the umbones, there 
being no appreciable sign of the sulcus and ridge which cross this portion of casts 
of that species. This difference is very obvious after it has once been pointed out. 
The beaks are also more strongly incurved and the hinge bent downward anteriorly 
in a greater degree, while the plate is probably also of less width. Finally, the 
posterior muscular scar is closer to the hinge and the longer diameter of the 
impressions more oblique. 
The systematic position of the species is near C. rotulata, C. cingulata, and C. 
tenella. The first is more rotund in outline, less oblique and has fuller umbones, 
the others are higher and have the beaks situated farther behind the anterior 
extremity. 
At Minneapolis C. persimilis is associated with a small Vanuxemia that is not 
easily distinguished unless the casts are clean and in good condition, The latter 
(V. decipiens,)difters somewhat in its outline being proportionally narrower anteriorly, 
but the principal difference lies in the character of the anterior adductor scar, which 
is much more distinct from the umbonal cavity. In short, the species is not a 
Crytodonta but a Vanuxemia as now defined. 
Formation and Locality.—Trenton limestones, Minneapolis, Minnesota, ‘‘ Lower Blue Beds” of the 
Trenton formation at Beloit, Wisconsin. 
Cyrtoponta ovirormis, Ulrich. 
PLATE XXXIX, FIG. 16; PLATE XL, FIG. 1. 
Cypricardites oviformis ULRICH, 1892. Amer. Geol., vol. x, p. 99. 
Shell rather above the medium in size, moderately convex, but little oblique, the 
outline almost.regularly oval, with the posterior end a little the widest and a slight 
straightening along the cardinal margin. Beaks small, situated between one-fourth 
and one-fifth of the length behind the anterior extremity; erect, compressed and not 
