1008 THE PALEONTOLOGY OF MINNESOTA. 



[Clathrosplra conlca. 



CLATHROSPIKA CONICA, n. sp. 



PLATE LXX, FIGS. 14. 



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 than 18 mm. The average width of the Cincinnati 

 form is somewhat greater, being something like 23 or 24 mm. Aside from the matter of size, the species 

 differ constantly from C. 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 C. 

 cornea 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 Kiver group (Ctenodonta bed 

 chiefly) at Minneapolis, St. Paul, Cannon Falls, Chatfleld, 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. Scofleld. 



to 



Genus PLETHOSPIRA, n. gen. (Ulrich.) 



In part Pleurotomaria and Holopea, the first of HALL, the second of WHITFIBLD. 



For generic diagonosis see page 958. 



This genus is proposed for more or less turbinate Holopea-\ike 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. Evtomaria 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 ap'erture 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. 



