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. Xamely, 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. 



MaduriUs, LESUEUR, 1818, Jour. Acad. Nat. Sci., vol. i, p. 312. 



Maclurea, WOODWARD, Manual of Shells, p. 202; EMMONS, 1842, Geol. Kept., p. 276; SALTER, 1859, 

 Can. Org. Rem., Decade 1, 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 laminae 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 Euomphalidce. 

 This view is supported by the fact that the lines of growth on the flat side of the 

 whorls, are usually sinuated, while a somewhat similar form of shell is characteristic 

 of Ophileta. Besides M. magna, we regard M. biysbyi Hall, M. loyani Salter, and 

 M. crenulata Billings, as thoroughly in accordance with the requirements of the 

 genus. 



