1008 THE PALEONTOLOGY OF MINNESOTA. 
(Clathrospira conica. 
CLATHROSPIRA CONICA, 2. Sp. 
PLATE LXX, FIGS. 1—4. 
This species has been, we believe, often confused with young specimens of C. subconica Hall sp. 
Though greatly resembling that species, especially in the condition of casts, and perhaps derived from 
it, it is nevertheless a well-marked form and worthy of a distinct name. It is always smaller, the largest 
of over fifty specimens being only 25 mm. wide, while in at least four-fifths of the specimens found in the 
Black River and Trenton groups the width is less than18mm. The average width of the Cincinnati 
form is somewhat greater, being something like 23o0r24mm. Aside from the matter of size, the species 
differ constantly from (. subconica in at least two particulars. First, the upper slope of the whorls is 
nowhere convex but is either gently concave throughout or flat from the linear suture to the rising base 
of the upper boundary of the peripheral band. The second difference lies in a more or less well-marked 
concave space which occupies the outer third of the base of the whorls. The inner border of this space is 
often very sharply defined on testiferous examples and readily traced on most casts of the interior. Among 
less obvious and perhaps less constant differences we may mention that the periphery is more angular, the 
whole base less convex, and the surface markings altogether less beautiful, less distinct and less regular. 
In C. subconica the lines of the surface sculpture are sharply raised and look like woven threads; in OC. 
conica they are neither sharp nor thread-like and generally require a good glass to bring them out at all. 
Formation and locality.—Not uncommon in the shales of the Black River group (Ctenodonta bed 
chiefly) at Minneapolis, St. Paul, Cannon Falls, Chatfield, and other localities in Minnesota. Also, but 
not so frequently, in the Clitambonites and Fusispira beds of the Trenton group at localities in Goodhue 
and Fillmore counties. In central Kentucky it occurs in the Trenton group; at Cincinnati, Ohio, in 
the Lorraine group. 
Collections.—E. O. Ulrich; W. H. Scofield. 
Genus PLETHOSPIRA, n. gen. (Ulrich.) 
In part Plewrotomaria and Holopea, the first of HALL, the second of WHITFIELD. 
For generic diagonosis see page 998. 
This genus is proposed for more or less turbinate Holopea-like shells, consisting 
of not more than five rapidly expanding ventricose whorls, with a broad, flat or 
concave, band situated near the middle of the whorls. There is no slit and the 
surface markings consist simply of lines of growth. None of the preceding genera 
are very closely related. Lophospira has angular whorls and a convex band, and 
represents quite a different line of development. Hvtomaria may be nearer, yet is quite 
as easily distinguished. Both Phanerotrema Fischer, and Bembexia Ehlert, possess a 
long apertural slit besides peculiarities of their own. The true position of Plethospira 
appears to be in the immediate vicinity of Hormotoma, Salter, aad Seelya, Ulrich. 
From the former it differs in having fewer and more rapidly enlarging volutions, 
and a shallower apertural notch, the forward curve of the lines of growth beneath 
the band, corresponding with the lower half of the outer lip, being much shorter 
than in Salter’s genus. ‘The lower extremity of the aperture also is much less 
produced and wider than is usual in that genus. Seelya includes very similar forms 
but they are easily distinguished by their different surface sculpture. 
