LAMELLIBRANCHIATA. 597 
Otenodonta madisonensis.] 
The concentric undulations also are more numerous. Ina paper soon to be published 
I shall propose the name Ctenodonta mundula for this small form. 
The three species mentioned in the preceding paragraph occupy an isolated 
position in the genus, and, though they may resemble some of the species of the 
levata section, I am satisfied that they are widely removed from them all. The 
shape of the anterior end is peculiar, as is also the reticulate surface ornamentation 
and the thin hinge plate with its oblique teeth. 
The species under consideration is named for Prof. Samuel Calvin, State Geolo- 
gist of Iowa. 
Formation and localityx—Maquoketa shales (Hudson River group), at Graf and other localities in 
northern Iowa. Also at Scale’s Mound in northwestern Illinois. 
Mus, Reg. No. 8628. 
CrENODONTA MADISONENSIS, ”. Sp. 
PLATE XLII, FIGS. 65 and 66. 
Shell subovate, slightly oblique, moderately convex, 12.5 to 15 mm. in length, 10 
to 12 mm. in hight, and 5.6 to 7 mm. in thickness; anterior end very short, rounded, 
posterior margin a little oblique, base broadly rounded and continuing into the 
anterior margin; dorsal outline slightly concave. Beaks anterior, small, scarcely 
prominent, incurved; posterior umbonal ridge rounded, inconspicuous; greatest con- 
vexity of valves in front and above the center. Surface almost smooth, only two or 
three obscure lines of growth having been noticed. Test rather thick, hinge strong, 
posterior denticles geniculated. 
This shell was included in this report and figured under the erroneous impres- 
sion that it represented a variety of C. calvini. Since the plates were prepared, 
however, another examination showed differences not before noticed, and when 
finally the shell was removed from one of the specimens so that a part of the hinge 
was uncovered, it became fully evident that it was not only distinct but belonged to 
quite another group of species. It is namely not far removed from such species as 
C. albertina and C, filistriata of this report, while it is especially near an unpublished 
form from the middle beds of the Cincinnati group in Kentucky and Ohio which I 
shall call C. tumida. From these three species C. madisonensis is distinguished by 
the more uniform curvature of the anterior margin, the antero-cardinal region in 
those forms being more or less prominent and subangular in outline. 
Formation and locality..—The specimens upon which the species is founded were collected in the 
Cincinnati group at Madison, Indiana, where they occurred in association with Ovthis retrosa, 
