956 THE PALEONTOLOGY OF MINNESOTA. 
(Synopsis of Pleurotomariidz. 
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, Pl. turbiniformis Meek and Worthen, 
and Pl. missouriensis Swallow. 
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 Huconospirw from the 
Pl. 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 Pl. filitexta, Pl. ella 
and Pl. 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, Pl. textiligera, in which the apical side of the shell agrees exactly 
with the Euconospire. 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 Pl. lucina and 
Euconospira turbiniformis is reasonably complete. 
The second line of development would begin in, say Pl. sulcomarginata Conrad 
of the Hamilton, and include Pl. nitella Hall, of the same formation, Pl. shumardi 
Meek and Worthen, of the Keokuk, and Pl. 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 Huconospire. 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. 
