956 THE PALEONTOLOGY OF MINNESOTA. 



[Synopsis of Pleurotomarildfe. 



the following turns; aperture oblique, the width much greater than the hight, the 

 inner lip scarcely thickened, the lower border, beginning at the inner extremity, 

 first convex then broadly concave and finally convex again when the edge turns 

 rather sharply backward to the slit which lies in the peripheral angle; upper margin 

 sweeping backward very strongly from the suture; length of slit equalling between 

 a third and a fourth of the last volution; band narrow, slightly truncating the 

 pheriphery, visible on all the volutions, concave, lying between sharply elevated 

 lines. Excepting the first three or four whorls, the surface is cancellated by fine 

 spiral lines crossing the lines of growth. Types, PI. turbiniformis Meek and Worthen, 

 and PI. missouriensis Swallow. 'V> i > /iWu4 atfaw <</& ^je^u^ >*_ ptAfad 



The deriviation of this genus is doubtful. There are two widely distinct 

 Devonian groups of species from either of which it may have been evolved. With 

 the evidence at hand, one derivation seems as plausible as the other, so we find 

 ourselves unable to decide for either. The first brings the Ewsonoapirce from the 

 PI. lucina group, in which the slit-band and surface markings are similar while the 

 form of the shell and of the volutions is very different. If this is the stock from 

 which the genus under consideration sprang, the first change consisted probably in 

 the gradual flattening of the dorsal surface of the whorls. In PI. filitexta, PI. ella 

 and PI. hebe, all species of the Hamilton group described by Hall, the necessary 

 conditions are supplied in increasing ratio. The next step is furnished by Meek's 

 Waverly species, PI. textiligera, in which the apical side of the shell agrees exactly 

 with the Euconospirce. The basal portion, however, is very different, being ventricose 

 instead of flat. Now, if we could find a shell having the convexity of the base 

 considerably reduced, we might say that the chain connecting PI. lucina and 

 Euconospira turbiniformis is reasonably complete. 



The second line of development would begin in, say PL sulcomarginata Conrad 

 of the Hamilton, and include PI. nitella Hall, of the same formation, PI. shumardi 

 Meek and Worthen, of the Keokuk, and PI. elegantula Hall, of the St. Louis. 

 The last species, though still too full at the base, nevertheless approaches very 

 nearly to the form pertaining to the Euconospirce. The principal feature lacking 

 is the spiral sculpture, which is wanting in all of the sulcomarginata or 

 Bembexia group. This difficulty, however, is lessened by the fact that the 

 spiral lines are wanting also on the first volutions of at least two species of 

 Euconospira. These early turns, furthermore, are slightly rounded on the dorsal 

 surface, thus strongly indicating that the type was evolved from another in which 

 the whorls were more rounded and without spiral markings. Taking all these 

 points into consideration, the balance of agreement seems to be in favor of the 

 second rather than the first line of development. 



