GASTROPODA. 1055 
Gyronema semicarinatum. ] 
upper slope, of which the inner third is almost horizontal and the outer two-thirds a strongly concave 
slope, a sharp angle or ridge dividing the two parts; second, the peripheral band, which is nearly vertical, 
margined on each side by a sharp carina and rendered somewhat bi-concave by a smaller carina, which 
first makes its appearance on the fifth whorl, lying slightly beneath the middle of the space; and third 
the basal part of the whorl is flattened rather than ventricose and traversed by five or six small revolving 
ridges and furrows, the last of which marks off the boundary of the small umbilical perforation. Surface 
showing very fine and rather regular lines of growth which cross the whorls from above somewhat 
obliquely backward. On one beautifully preserved example the whole surface, when highly magnified, is 
seen to be covered with a crowded set of extremely delicate revolving lines. Aperture subovate or some- 
what hexagonal, its hight slightly exceeding the width; peritreme thin, the inner lip reflected partly over 
the umbilicus. 
Formation and locality—Ctenodonta bed of the shales of the Black River group, Minneapolis, Chat- 
fleld and near Cannon Falls, Minnesota; also in the limestones of this group in Mercer county, Kentucky. 
Collections.—Geological and Natural History Survey of Minnesota; E. O. Ulrich. 
Museum Register, No. 6854. 
GYRONEMA SEMICARINATUM Salter. 
PLATE LXXVIII, FIGS. 17 and 18. 
Cyclonema semicarinata SALTER, 1859, Can. Org. Rem., Dec. I, p. 27, pl. vi, figs. 2 and 2a, not 2b. 
This species is distinguished from G. pulchellum by its stronger lines of growth, and the much 
greater prominence of the carina which corresponds with the upper boundary of the vertical peripheral 
band in that species. The lower boundary is only very slightly more prominent than the rest of the six 
or seven revolving carine which lie beneath the principal keel. The upper slope of the whorls also is 
- somewhat wider than in G. pulchellum, and, furthermore, may be, as in the specimen figured, divided 
on the last whorl by an extra small median carina. This extra carina is probably not of much conse- 
quence since it occurs only on a part of the last whorl in our specimen, and is neither mentioned nor illus- 
trated by Salter. 
We have not the least doubt concerning the specific identity of the Minnesota specimen and the 
shell represented by Salter’s figures 2 and 2a (loc. cit.), but we cannot say as much for his fig. 2b. The last 
might pass very well for our G. pulchellum since it gives the peripheral band as nearly vertical, the carina 
which forms its lower boundary being nearly or quite as prominent as the upper. The upper slope, how- 
ever, is not as concave as it should be in our species. 
Formation and locality.—Phylloporina bed of the shales of the Black River group, near Cannon Falls, 
Minnesota. 
Collection.—H, O. Ulrich. 
GYRONEMA DUPLICATUM, 7. Sp. 
PLATE LXXVIII, FIGS. 22—25. 
Comp. Pleurotomaria percarinata HALL, 1847, Pal. N. Y., vol. i, p. 177, pl. XXXVIII, fig. 4. 
This is distinguished from G. pulchellum, which it resembles in a general way much more closely 
than it does G. semicarinatum Salter sp., by its larger size, entirely closed umbilicus, and less angular 
whorls. ‘The carina on the upper slope of the whorls is less prominent and smaller (in casts of the interior 
it looks more like a thick swelling than a carina), and on the last whorl it is apparently always divided 
into two thin lines. Between these and the upper peripheral angle, which is the strongest but not the 
most prominent on the whorl, the surface is concave, but not as deeply so as in G. pulchellum. The lower 
keel of the peripheral band is not always readily distinguished since it is followed on the slightly ventri- 
cose base by six or seven similar though generally somewhat smaller carinz, while above it there is at 
