596 THE PALEONTOLOGY OF MINNESOTA. 
[Ctenodonta calvini. 
hinge, with its oblique teeth, is very different. In C. socialis the outline is more 
regularly oval and the beaks situated as much behind the center as they are in front 
of that point in C. fecunda. In C. simulatrix the anterior end is wider, the muscular 
scars deeper and the hinge more numerously denticulated. Similar differences dis- 
tinguish C. albertina, a well marked species having also a stronger hinge and genicu- 
lated teeth. The following species, C. calvini, is probably nearer than any other. 
species now known. 
Formation and locality.—Very abundant in the lower so-called Maquoketa shales near Dubuque, 
Iowa; Scale’s Mound, [llinois, and Platte’s Mound in Lafayette county, Wisconsin. The species occurs 
also in Fillmore county, Minnesota, in equivalent beds (Hudson River group), though but rarely. 
CTENODONTA CALVINI, ”. Sp. 
PLATE XLII, FIGS. 61—64. 
Shell subquadrate-ovate, about 15 mm. long, 12 mm. high and 6 mm. thick; 
anterior end rounded, a trifle narrower than the slightly truncate posterior end; 
the latter is a little oblique, gently convex except below, where the outline turns 
rather sharply into the broadly rounded base; above it forms an obtusely angular or 
rounded junction with the straight post-cardinal margin; in front of the scarcely 
prominent beaks, which are situated about one-third of the entire length behind the 
anterior extremity, the outline is more or less concave. Surface of valves rather 
uniformly convex, with the posterior umbonal ridge strongly rounded, though in no 
case conspicuous. External surface markings not observed. Casts of the interior 
exhibit a few concentric undulations, and in the central and ventral parts a variable 
uumber of obscure rays. The test seems to have been unusually thin. Hinge plate 
very narrow, bent at the beak, straight behind, gently concave in front; denticles 
small, oblique, about sixteen posterior and eight or ten anterior in each valve. 
Adductor muscular scars very slightly impressed, the posterior one extended above, 
larger and longer than the anterior, and placed in the middle of the cardinal slope, 
so that its long axis is parallel with the umbonal ridge; several small umbonal scars 
may be observed. 
This fine species, though closely related, is at once distinguished from C. fecunda 
by its greater posterior hight and larger size. The convexity of the valves also is 
somewhat less, and other differences may be detected in comparing the figures 
of the two species on plate xu. A nearer form, perhaps it ought to be called a 
variety, occurs in the lower beds (Utica horizon) of the Cincinnati group at Coving- 
ton, Kentucky. This has exceedingly fine and crowded concentric strie, crossed by 
more distant radiating lines. Casts of the interior have about the same shape as 
C. calvini, but they are all much smaller, the largest having a length of only 7 mm. 
