GASTROPODA. 1041 
Maclurina.] 
comparison it will be observed that the umbilicus does not descend as abruptly as in that species, the 
convex slope being much more inclined giving a relatively greater width to the umbilicus at its narrowly 
rounded margin. 
Var MACRA, n. var. 
PLATE LXXV, FIGS. 15 and 16, 
The cast upon which this variety is based, was found at a lower horizon than that which holds the 
typical form. So far as we can see it differs only in being more depressed, the transverse and vertical 
diameters of the last whorl near the aperture being to each other respectively as four is to three. In the 
typical form the same dimensions are as four is to five. Part of this difference may be due to distortion— 
indeed we think it is, since at the inner end of the outer whorl the two dimensions are almost equal. 
Formation and locality. Of the typical form we have four specimens, from the Maclurea bed of the 
Trenton group, of which one was obtained from each of the following lozalities in Minnesota: Lime City, 
Stewartville, Pleasant Grove and Wykoff. One of these belongs to the Survey museum, the others to E. 
O. Ulrich. Var. macra was collected by the latter in the Fusispira bed at Hader. 
Museum Register, No. 8442. 
Genus MACLURINA, n. gen. 
This genus is proposed in accordance with our remarks on page 1038, for the 
reception of shells heretofore classed as Maclurea, but differing from the typical 
form of the genus in wanting the projections for the attachment of muscles on 
the inner side of the operculum. Maclurea manitobensis Whiteaves, the operculum 
of which is figured and described by Whiteaves in the Canadian Record of Science 
for April, 1893, is regarded as the type of the new genus. In this species the 
nucleus is at the junction of the lower and inner margin of the operculum, and 
we believe the same is true of M. cuneata and M. subrotunda of Whitfield, which 
with Whiteaves’ species, are all that at the present time it seems safe to refer 
to Maclurina. Billings says of the operculum of his M. oceana that it has no 
muscular process, and he also figures the opercula of two otherwise unknown 
species which likewise are without such projections. But these opercula differ 
so widely from that of M. manitobensis that it seems highly improbable that they 
can belong to shells of the same genus. 
Maciurina MANITOBENSIS Whiteaves. 
PLATE LXXYVI, FIGS. 4 and 5; PLATE LXXXII, FIG. 45. 
Maclurea manitobensis WHITEAVES, 1890, Trans. Roy. Soc. Can., vol. vii, Sect. 4, p. 75; also 1893, 
Canadian Record of Science, p. 324. 
Original description.—‘‘Shell large, attaining to a maximum diameter of eight inches and a half, and 
consisting of about five somewhat slender volutions which increase rather slowly in size; outer volution 
nearly always distinctly angulated at the periphery. Left (under) side almost flat, but faintly depressed 
in the center in some specimens and as faintly raised in others; volution, as viewed on the flat side, very 
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