GASTROPODA. 1039 
Maclurea bigsbyi.] 
Macturea siassyt Hall. 
PLATE LXXV, FIGS. 5—10. 
Maclurea bigsbyi HALL, 1861, Geol. Rep. Wis., p. 37; WHITFIELD, 1882, Geol. of Wis., vol iv, p. 222, 
pl. vi, figs. 17 and 18; also 1895, Mem. Am. Mus. Nat. Hist., vol. i, p. 62, 
pl. vi, figs. 14 and 15 (not 12 and 13). 
Shell of medium size, ranging in diameter from 25 to 80 mm., usually 50 to 60 mm.; normal hight 
varying from five-twelfths to one-half of the width. Lower surface flat, the outer angle subacute; in 
casts more or less obtuse, and with the inner whorls somewhat rounded and the sutures generally much 
more distinct than on the shell itself. Umbilicus deep, rather abrupt, exposing half or more of each of 
the inner whorls, its width always greater than one-third of, and sometimes exceeding half the width 
of the entire shell; margin of umbilicus angular in shells, abruptly rounded in casts, the slopes convex on 
each whorl. Casts rarely retain traces of the surface markings. These consist of more or less obscure and 
irregular transverse lines and undulations, crossed on the peripheral region only by revolving lines a mm, 
or less apart. Operculum much as in I, logani, except that the nucleus is at the lower inner angle instead 
of in the center of the lower side. 
The original localities for this species afford two closely allied species, a small one that is about 25 
mm. in diameter, and a larger form that commonly is more than twice as wide. These two forms differ 
further in the relative size of the umbilicus, its width in the smaller form being in no observed case more 
than one-third of the width of the shell, while in the larger form it generally equals one-half. Under the 
circumstances it is highly probable that both forms were included by Hall in his M. bigsbyi, and it is a 
matter of considerable difficulty to decide as to which of the two is the better entitled to retain his name. 
The larger species being the more common and widely distributed, and undoubtedly the same as the best 
preserved and largest of the original types of the species figured by Whitfield in 1895, we have decided to 
restrict the application of the specific name bigsbyi to it, and to propose the new name nitida for the other. 
Formation and locality —Stones River group, Beloit, Janesville, Mineral Point, and other localities 
in southern Wisconsin; Dixon and La Salle, in Illinois; Lebanon and near Knoxville, Tennessee. Dr. 
F. W. Sardeson catalogues the species as occurring in the Vanuxemia bed in Minnesota, but we have not 
been able to verify its occurrence in the state. 
Collections. —Geological and Natural History Survey of Minnesota; E. O. Ulrich. 
Museum Register, Nos. 7308, 7349. 
MacLurREA BIGSBYI Var. DIXONENSIS n. var., and MacturEA KNOXVILLENSIS 
n. sp. (Ulrich.) 
(Not figured.) 
Two other species of this type, both three inches or more in diameter, are known to us, in the one 
case from rocks holding M. bigsby2, in the other in strata supposed to be equivalent or nearly so. The first, 
which was collected at Dixon, Illinois, is more depressed and has the margin of the umbilicus, which 
takes up about one-third of the diameter of the shell, moved farther inward so that its walls are almost 
vertical. As the form is easily recognized, we suggest the provisional designation M. bigsbyi var. dixonensis. 
The other was received from Prof. J. M. Safford, who coliected the shell and opercula in the vicinity of 
Knoxville, Tennessee. As it deserves to rank as a distinct species, we propose the name Maclurea knox- 
villensis. With a general aspect like M. bigsbyt, it differs decidedly in having less angular whorls and deep 
sutures on the flat side. The operculum, so far as the position of the nucleus is concerned, is more like 
that of M, logani than M. bigsbyi, but differs strongly from both in the fact that the nucleus is extremely 
prominent and twisted, recalling,”somewhat feebly, a small ram’s horn. 
