1010 THE PALEONTOLOGY OF MINNESOTA. 



IPlethosplra semele. 



PLETHOSPIRA SEMELE Hall. 



PLATE LXX, FIGS. 8-10. 



Pleurotomaria semele HALL, 1861, Geol. Rep. Wis., p. 36; WHITFIELD, 1895, Memoirs of the Arner. 

 Mus. Nat. Hist., vol. i, pt. 2, p. 61, pi. vm, flgs. 8-10. 



Original description." Shell subconical ; spire ascending ; bight 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 strife, which are curved abruptly backwards from the suture to the revolving 

 band, on which they make a shallow retral curve, and below the band have a gentle forward curvature 

 in passing downward to the umbilicus. Hight a little 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. Whitfleld's figure 10 (loc. 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 Whitfleld's figure, however, they are represented 

 as curving very strongly forward. The latter shows also an angular bend in the stria; 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 Whitfleld'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 striae is, we believe, accurately represented in our figures 8 and 9. 



Whitfleld'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 interioT 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 

 bicincta. 



Formation and locality. Utica group (Maquoketa shales), Maquoketa creek and Graf, Iowa. 

 Collections. Geological and Natural History Survey of Minnesota; E. O. Dlrich. 

 Museum Register, No. 7334. 



