1042 THE PALEONTOLOGY OF MINNESOTA. 



LMaclurina cuneata. 



shallowly concave in the center and slightly raised on the outer margins ; suture lightly impressed. Right 

 (upper) side moderately convex (the greatest thickness or depth varying in different examples from two- 

 flfths to one-third the maximum diameter); somewhat conical or subhemispherical, the outer volution 

 obliquely flattened and narrowing very rapidly, but in a few specimens somewhat convexly, from the 

 periphery to the umbilical margin ; umbilicus deep, conical, and apparently about equal in breadth to one- 

 fourth of the maximum diameter, though in all the specimens collected, the test is either imperfect or 

 absent at the umbilical margin ; aperture obliquely and rather narrowly subtrapeziform ; outer lip appar- 

 ently simple ; test thick. 



" Surface of the test on the left or flat side marked with irregularly disposed, but for the most part 

 distant, transverse linear grooves or periodic arrests of growth, each of which curve gently backward in a 

 very shallowly convex curve, and occasionally with a few striations which run parallel to them. In one 

 of the specimens figured which is a little less than four inches in its greatest diameter, and in which the 

 whole of the test is preserved on the flat side, there are six of these periodic arrests of growth on the 

 outer volution, while the inner whorls are perfectly smooth. In larger but similarly preserved specimens, 

 these arrests of growth which are not sufficiently deep to produce any impression on the casts, are some- 

 what more numerous and disposed at still more unequal intervals. On the right or convex side the test 

 is ornamented with rounded spiral ribs of nearly equal size, and these are crossed by similarly shaped, 

 straight and transverse cost, in such a way as to present a somewhat nodulous appearance. The spiral 

 ribs, however, seem to be rather broader than the narrow furrows between them while the transverse 

 costae are apparently equal in breadth to the regularly concave grooves which alternate with them." 



The Minnesota specimens referred to this species are all, save one, mere casts of the interior, but 

 they agree so well with Whiteaves' figures and descriptions that there is little room for doubt concerning 

 their identity with the Manitoba types of the species. The excepted specimen preserves the shell on the 

 inner whorls only, and shows that the first three or four turns are very small and rounded on the lower side. 

 The specimen has a width of about four inches, and consists of nearly six whorls, at which rate the largest 

 example should make about seven complete volutions. None of the examples seen retain any of the test 

 of the convex side of the shell, nor have we seen good moulds of the exterior, hence we cannot say that 

 they had revolving lines like those found on one of the specimens described by Whiteaves. 



Formation and locality. Maclurea bed of the Trenton group. Whiteaves (toe. cit.) mentions numer- 

 ous localities in Manitoba. In Minnesota the species is not as abundant as the associated M. cuneata, but 

 may be found at most localities in Goodhue, Dodge, Olmsted and Fillmore counties, where its particular 

 horizon is exposed. 



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

 Museum Eegister, No. 4105. 



MACLURINA CUNEATA Whitfield. 



PLATE LXXVI, FIGS. 1-3; PLATE LXXX1I, FIG. 48. 



| 



Maclurea cuneotn WHITFIELD, 1878, Ann. Rep. Geol. Sur. Wis., p. 75; and 1882, Geol. Wis., vol. iv, p. 

 246, pi. ix, figs. 5-6. 



The diameter in this species, as far as known, does not exceed three inches, while the umbilical 

 perforation is very small; otherwise casts of the interior agree almost exactly with those of M. manitobensis. 

 A single testiferous example belonging to Mr. Ulrich's collection has been observed. It is a very small 

 specimen, being only 16 mm. in diameter, and embedded in the rock, but on being ground down so as to 

 show a vertical section, it brings out as shown in fig. 46, pi. LXXXII, soino interesting features. The 

 specimen consists of three and a half whorls, all gently convex on the lower side but differing considerably 

 in other respects. The first two and a half turns are coiled nearly in the same plane, so that they are 

 almost entirely exposed on the upper or umbilical side. With the next turn, however, the umbilicus is 

 greatly contracted and in the following half turn the normal or rather the specific characters of the shell 

 are established. 



