538 THE PALEONTOLOGY OF MINNESOTA. 
: [Cyrtodonta ampla. 
of moderate strength; cardinal teeth three in each valve, sub-equal, curved and 
rather oblique; posterior teeth slender, two or three in each valve. Surface of shell 
with somewhat irregular concentric lines of growth. No trace of these are to be 
seen in casts of the interior. : 
It is possible that the casts above described really belong to C. huronensis. 
Although I have compared them with an authentic example of that species, labelled 
by Billings himself as from the original locality for the species, I could not satisfy 
myself. The Wisconsin casts are certainly distinct from this specimen, having 
smaller umbones and shorter anterior end, but the latter also does not agree with 
Billings’ figures. Very likely the illustrations are not entirely trustworthy. 
Compared with C. subovata, the species is distinguished by its shorter, narrower, 
and less distinct anterior end, comparatively greater length, less produced and more 
oblique cardinal teeth, and more distinct muscular and pallial impressions. That 
species also attains greater size. 
Formation and lacality.—‘t Lower Blue beds” of the Trenton at Janesville and Beloit, Wisconsin. 
Mus. Reg. No. 8323. 
CYRTODONTA AMPLA, %. Sp. 
PLATE XXXIX, FIG. 34. 
In the outline this species resembles C. subovata and C. janesvillensis very closely. 
It is known only from casts, but these are distinguished at once by the oblique ridge 
running from the beak toward the posterior third of the base. On the anterior side 
the surface descends sharply from the ridge into an unusually wide flattened space. 
C. janesvillensis is also narrower anteriorly and relatively more convex. Another 
species with which it is to be compared is the Galena form described by Meek and 
Worthen as C. obliqua. The outline of that species is different being narrower in 
. front and more produced in the postero-basal region, giving the shell a more erect 
appearance. Its valves are also a little more convex. C. glabella is shorter. In the 
associated forms of Vanuxemia the anterior adductor scar is much more sharply 
defined. 
Formation and locality.x—Treuton limestone, Cannon Falls, Minnesota. 
CYRTODONTA BILLINGSI, 2. Sp. 
PLATE XL, FIGS. 2—6. 
Cypricardites ventricosus Whitfield, 1882 Geol. Wis., vol. iv, p. 209, pl. 5, fig. 9. 
Shell of medium size or less, transverse, obliquely ovate, highest in the posterior 
half; valves strongly ventricose in the umbonal and central regions. Hinge line 
at least two-thirds the length of the shell, slightly arcuate, posteriorly declining 
