1010 THE PALEONTOLOGY OF MINNESOTA‘ 
|Plethospira semele. 
PLETHOSPIRA SEMELE Hall. 
PLATE LXX, FIGS. 8-10. 
Pleurotomaria semele HALL, 1861, Geol. Rep. Wis., p. 36; WHITFIELD, 1895, Memoirs of the Amer. 
Mus. Nat. Hist., vol. i, pt. 2, p. 61, pl. vu, figs. 8-10. 
Original description.—‘‘ Shell subconical; spire ascending; hight and breadth nearly equal, consist- 
ing of four or five rounded or subangular volutions, the last one ventricose, subangular on the periphery, 
regularly rounded below into the small umbilicus. Aperture round. Surface marked by a subangular 
carina a little below the suture, and on the periphery by a moderately broad revolving band, sharply ele- 
vated at the margins and concave in the middle. Entire surface marked by sharp, elevated, closely 
arranged, concentric strie#, which are curved abruptly backwards from the suture to the revolving 
band, on which they make a shallow retral curve, and beiow the band havea gentle forward curvature 
in passing downward to the umbilicus. Hight alittle more than one inch; width three-fourths of an 
inch.” 
We have two specimens of this species besides an interior cast that may belong to some other 
species. One of these (the specimen represented by figures 8 and 9 on plate LXX) retains considerable of 
the shell with its markings, and permits us to add several particulars to the above description. First, 
there is a slight angle or ridge, about midway between the peripheral band and the margin of the umbil- 
icus, that shows only on the last whorl, being just covered in the preceding whorls. The space above it 
is flat or a trifle concave. Next, the umbilical perforation is very small and partly covered by the reflexed 
columellar lip. Finally, the lines of growth are quite regular, with between two and three in the space 
of one millimeter. The apical angle is about 65°. 
We cannot understand how Prof. Whitfield’s figure 10 (Joc. cit.) can possibly be a true portrayal of 
the surface markings of the original type of the species. Certain it is that it does not give them as they 
are in the specimens which we have identified with P. semele. Nor does it agree with Hall’s description, 
while our specimens do. Hall says that the lines of growth below the band ‘‘have a gentle forward 
curvature in passing downward to the umbilicus.” In Whitflield’s figure, however, they are represented 
as curving very strongly forward. The latter shows also an angular bend in the strie at the upper carina 
which is not mentioned by Hall, and which is, to say the least, a very rare condition among shells of this 
family. Now, either Whitfield’s illustration is wrong or Hall’s description, and as our specimens agree 
with the latter, we are obliged to believe that the fault lies with the figure. The course, as well as the 
character of the striz is, we believe, accurately represented in our figures 8 and 9. 
Whitfield’s remark that ‘there is no doubt that this shell is a true Murchisonia, unless it prove to 
be a Lophospira,” strikes us as a very peculiar statement. He proposed Lophospira and should therefore 
know when he has a species of his own genus before him and when one of Murchisonia. However that 
may be, P. semele most certainly belongs to neither of those genera. As we have said on a previous page, 
the occurrence of a true Murchisonia in American strata is yet to be established. As to Lophospira, it has a 
convex band, and includes only shells having strongly angular whorls, while P. semele has rather rounded 
whorls and is a species of the concave-banded types. : : 
Concerning the original of our figure 10, we wish to say that if it really is an interior cast of a shell 
of this species, and a small patch of shell on the under surface, as well as a flattening of the peripheral 
band near the mouth indicates that it is, then casts are sometimes dangerously like casts of Lophospira 
bicineta. 
Formation and locality —Utica group (Maquoketa shales), Maquoketa creek and Graf, Iowa. 
Collections.—Geological and Natural History Survey of Minnesota; HE. O. Ulrich. 
Museum Register, No. 7334. 
