1038 THE PALEONTOLOGY OF MINNESOTA. 
[Maclurea. 
a provisional measure. We admit that the time has not yet arrived when it will be 
possible to divide the whole genus into natural groups, but convenience demands 
that at least one section should be distinguished now. All definitions of Maclurea 
give, as perhaps the most essential feature, one character that is known to be absent 
in several species, which nevertheless are always classified without question under 
the Maclurea. Namely, two more or less prominent muscular scars or processes upon 
inner side of the operculum. As the absence of these projections in certain species 
certainly deserves some recognition in our classification, we propose to separate them 
as a distinct genus, under the new name Maclurina, which we have selected in 
order to facilitate recollection of their previous generic association. In breaking 
up the genus we deem it advisable to proceed with extreme caution, since the 
opercula, upon which the division rests, are fully known in only a few cases. We 
shall, therefore, change the generic designation at the present time in only three 
instances, and leave the arrangement of the remainder for future investigation. 
Genus MACLUREA, (Lesueur.) Woodward. 
Maclurites, LESUEUR, 1818, Jour. Acad. Nat. Sci., vol. i, p. 312. 
Maclurea, WoopWARD, Manual of Shells, p. 202; Emmons, 1842, Geol. Rept., p. 276; SALTER, 1859, 
Can. Org. Rem., Decade ], p. 7. 
Shell thick, discoidal, few whorled, reversed, the under side flat or nearly so 
and exposing all the whorls, the upper side convex and deeply perforated in the 
center instead of raised into a spire; surface with lines of growth crossing the 
whorls almost directly, the peripheral portions not infrequently exhibiting a 
revolving set of lines also. Operculum more or less curved in a front view, set 
somewhat obliquely into the aperture, and made up of concentric lamine with the 
nucleus, which is in the middle or near the outer angle of its lower part, projecting 
more or less forward, sometimes like a great vertically compressed horn; inner side 
excavated, with a prominent projection for the attachment of a muscle in the lower 
inner fourth of the excavation and a large muscular scar, little or not at all elevated, 
in the upper inner fourth. Type: Maclurea magna Lesueur. 
It may be that the shells of this genus are, as supposed by Billings and others, 
really sinistral, in which case the flat side would be the spire and the umbilicated 
side the base, but we prefer for the present, to regard the ridge which usually 
surrounds the umbilicus as corresponding to the notch-keel of the Hwomphalide. 
This view is supported by the fact that the lines of growth on the flat side of the 
whorls, are usually sinuated, while asomewhat similar form of shell is characteristic 
of Ophileta. Besides M. magna, we regard M. bigsbyi Hall, M. logani Salter, and 
M. crenulata Billings, as thoroughly in accordance with the requirements of the 
genus. 
