1040 THE PALEONTOLOGY OF MINNESOTA. 



[Maclurea nltida 



MACLUBEA NITIDA, n. sp. 



(Perhaps a small variety of Af. l>\Qli)il Hall.) 

 PLATE LXXV, FIG. 11. 



Maclurea bigsbyi (part) HALL, and WHITFIELD. (See description of that shell on page 1039.) 



Shell rather small, 20 to 30 mm. wide, half as high, and resembling the young of M. bigsbyi in all 

 respects save that (1) the umbilicus is narrower, in no case exceeding a third of the width of the shell, (2) 

 less sharply defined, the turn into the umbilicus being abruptly rounded but never angular, (3) the trans- 

 verse striae between the umbilicus and periphery more regular and sharper, and (4) the transverse section 

 of the whorls (see plate LXXV, fig. 9) a little different, the night being relatively a trifle greater, and the 

 section less obviously triangular. Remains of three or four revolving lines occur on the periphery of one 

 specimen, but nothing of the kind is visible on any of the others. 



Formation and locality. Stones Eiver group, Mineral Point and Beloit, Wisconsin; Dixon, Illinois, 

 and Murfreesboro, Tennessee. 



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

 Museum Register, No. 7356. 



MAOLUREA DEPRESSA, n. sp. 



PLATE LXXV, FIGS. 1-4. 



Shell of medium si/e, depressed, about 55 mm. wide, and 18 mm. high at the aperture ; under side of 

 whorls more or less distinctly concave, the outer and inner edges being somewhat elevated; inner edge form- 

 ing a sharp ridge in casts of the interior; umbilical perforation abrupt, comparatively small though show- 

 ing all the inner whorls, its width equalling less than a third of the diameter of the shell. Surface 

 markings apparently as in M. bigsbyi Hall. 



The concave under surface of the whorls, more depressed form, and smaller and more abruptly 

 descending umbilicus are the differences relied on in distinguishing this species from M. bigsbyi. Variety 

 dixoneni* of that species, which might perhaps with better propriety be classed either as an intermediate 

 species or as a variety of N. depressa, differs chiefly, if not solely, in the form of the under side of the 

 whorls, these being flat or gently convex, as in M. bigsbyi, instead of concave. 



Formation and locality.- Stones River group, Minneapolis, Minnesota. 

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

 Museum Register, No. 6858. 



MACLUREA CRASSA, n. sp., and var. MACRA, n. var, 



PLATE LXXV, FIGS. 12-16. 



Externally this species resembles M. bigsbyi, but, aside from the fact that it is a much heavier shell 

 (on the outer and under sides of the last whorl it varies in thickness between the extremes of 3 mm. and 

 6 mm.), it differs in having the inner whorls convex and slightly elevated on the under side, the outer or 

 peripheral angle more obtuse, and the umbilical depression wider. The width of the latter is to the 

 diameter of the shell as 32 is to 58, the numbers representing the respective dimensions in millemeters of 

 a testlferous example. Comparing casts of the interior of the two species, the differences are more 

 obvious, the whorls of M. crassa being more slender and more rounded in section on the lower side 

 especially, and the umbilicus open to such an extent that nearly the whole width of the inner whorl is 

 exposed to view. Because of the extreme thickness of the shell, the suture, though very close on the 

 exterior, is unusually deep in casts, while the mouth expands somewhat like a trumpet. Casts of the 

 interior look very much like the exterior of the shell of M. knoxeillenns; yet even with this unequal 



