1040 THE PALEONTOLOGY OF MINNESOTA. 
[Maclurea nitida. 
MACLUREA NITIDA, %. Sp. 
(Perhaps a small variety of M. bigsbyi Hall.) 
PLATE LXXV, FIG. 11. 
Maclurea bigsbyi (part) HALL, and WHITFIELD. (See description of that shell on page 1039.) 
Shell rather small, 20 to 30 mm. wide, half as high, and resembling the young of WM. bigsbyi in all 
respects save that (1) the umbilicus is narrower, in no case exceeding a third of the width of the shell, (2) 
less sharply defined, the turn into the umbilicus being abruptly rounded but never angular, (3) the trans- 
verse stria- between the umbilicus and periphery more regular and sharper, and (4) the transverse section 
of the whorls (see plate LX XV, fig. 9) a little different, the hight being relatively a trifle greater, and the 
section less obviously triangular. Remains of three or four revolving lines occur on the periphery of one 
specimen, but nothing of the kind is visible on any of the others. 
Formation and locality.—Stones River group, Mineral Point and Beloit, Wisconsin; Dixon, Illinois, 
and Murfreesboro, Tennessee. 
Collections.—Geological and Natural History Survey of Minnesota; EH. O. Ulrich. 
Museum Register, No. 7356. 
MACLUREA DEPRESSA, 7%. sp. 
PLATE LXXV, FIGS. 1—4. 
Shell of medium size, depressed, about 55 mm. wide, and 18 mm. high at the aperture; under side of 
whorls more or less distinctly concave, the outer and inner edges being somewhat elevated; inner edge form- 
ing asharp ridge in casts of the interior: umbilical perforation abrupt, comparatively small though show- 
ing all the inner whorls, its width equalling less than a third of the diameter of the shell. Surface 
markings apparently as in M, bigsbyi Hall. 
The concave under surface of the whorls, more depressed form, and smaller and more abruptly 
descending umbilicus are the differences relied on in distinguishing this species from M. bigsbyi. Variety 
dixonenis of that species, which might perhaps with better propriety be classed either as an intermediate 
species or as a variety of M. depressa, differs chiefly, if not solely, in the form of the under side of the 
whorls, these being flat or gently convex, asin M. bigsbyi, instead of concave. 
Formation and locality.— Stones River group, Minneapolis, Minnesota. 
Collections.—Geological and Natural History Survey of Minnesota; E. O. Ulrich. 
Museum Register, No. 6858. 
MAcLUREA ORASSA, ”. Sp. and Var. MAORA, ”. var. 
PLATE LXXV, FIGS. 12—16, 
Externally this species resembles M. bigsbyi, but, aside from the fact that it isa much heavier shell 
(on the outer and under sides of the last whorl it varies in thickness between the extremes of 3mm. and 
6 mm.), it differs in having the inner whorls convex and slightly elevated on the under side, the outer or 
peripheral angle more obtuse, and the umbilical depression wider. The width of the latter is to the 
diameter of the shell as 32 is to 58, the numbers representing the respective dimensions in millemeters of 
a testiferous example. Comparing casts of the interior of the two species, the differences are more 
obvious, the whorls of MM. crassa being more slender and more rounded in section on the lower side 
especially, and the umbilicus open to such an extent that nearly the whole width of the inner whorl is 
exposed to view. Because of the extreme thickness of the shell, the suture. though very close on the 
exterior, is unusually deep in casts, while the mouth expands somewhat like a trumpet. Casts of the 
interior look very much like the exterior of the shell of M. knoxvillensis; yet even with this unequal 
