568 THE PALEONTOLOGY OF MINNESOTA. 
[Whitella compressa. 
The great prominence and sharpness of the umbonal ridge, the decided flattening 
of the postero-dorsal region and the narrowness of the posterior extremity are the 
features that distinguish the species from all the others referred to the genus, except 
W. hindi Billings sp., W. carinata Meek sp., and W. truncata Ulrich. The first of these 
exceptions is not so high, less gibbous, less oblique, has a more prominent and less 
broadly rounded anterior side, straighter posterior margin, narrower beaks and a 
cardinal area or escutcheon that is a little longer but not nearly so wide. The other 
two are sufficiently distinguished by their much smaller size. 
The specimen described by Meek in the Ohio Paleontology (oc. cit.) and doubt- 
fully referred to this species is certainly distinct. It my belong to W. hindi Billings, 
or to W.umbonata Ulrich, both of which it resembles more closely than W. sterlingensis, 
especially in the prominence of the anterior end, which of itself precludes all possi- 
bility of its identity with the present species. That it really belongs to one or the 
other of the two species mentioned it would not now be safe to say, since I have no 
means of learning to what extent the specimen may have suffered from compression. 
Formation and locality,—The type specimen was found in the upper beds of the Cincinnati group 
at Sterling, Illinois. A small distorted shell from the Hudson River group near Spring Valley, Min- 
uesota, may belong here, but I cannot say as much for any specimen seen from the equivalent strata 
of Ohio and Indiana, despite the fact that the species is commonly believed to occur there. 
WHiITELLA compressa Ulrich. 
PLATE XLI, FIGS. 6—9, 
Whitella compressa ULRICH, 1890. Amer. Geol., vol. vi, p. 180. 
This shell has an outline very similar to that of W. obliquata, yet differs con- 
spicuously from that species in having much less gibbous valves, the thickness in 
that species equalling about one-half of its greatest length, while in W. compressa 
the length is more than two and one-half times the convexity. And yet the length 
of the latter is comparatively a little less than in the Hudson River group species. 
Comparing the two species critically we find further that in W. compressa the umbonal 
ridge is much less developed and the outline at the extremities of the hinge some- 
what different, the posterior part being a little more sharply rounded, while anteri- 
orly the hinge projects farther beyond the beaks and in a straighter line, so as to 
form an angular junction with the anterior margin. An undescribed form found 
associated with W. obliquata in Obio, and which I shall call W. ohioensis, attains a 
greater size, but agrees in all its specific characters much more closely with the 
present species. Indeed the agreement is so close that we may be justified in 
regarding it as a reapparition of W. compressa, the only difference so far detected 
with certainty being a slight one in the outline. The Ohio form, namely, is a little 
narrower across the posterior half of the shell, I expect, however, that when more 
