96 
AUG. F. FOERSTE 
bears the greatest resemblance, but in all cases the slit-band 
remains exposed, immediately above the sutures. 
Pleurotomaria helicina Lindstrom,^^ is similar in having the 
slit-band of earlier volutions covered up by the successively 
later volutions, but the slit-band is located slightly above mid- 
height of the volutions and there is a revolving callous thickening 
along the rim of the umbilicus. The shell is much more de- 
pressed. Possibly this species ma^^ prove congeneric with 
Cryptaulus fiUtextus. 
In the genus Trepospira Ulrich^^ the slit-band also is visible 
only on the last volution, but the umbilicus is closed by a callous 
deposit. Shells of this type originated apparently as early as 
the Hamilton formation, where Pleurotomaria rotalia occurs. 
However, the typical forms of this genus are of Carboniferous 
age. 
Lophospira (Ruedemannia?) inexpectans (Hall and Whitfield) 
Pleurotomaria inexpectans Hall and Whitfield, Geol. Sury. 
Ohio, Pal. 2, 1875, p. 117, pi. 5, fig. 12. 
Height 26.5 mm., greatest lateral diameter 23.5 mm., greatest 
height of spire 16.5 mm., apical angle 83°. Slit-band peripheral, 
1.5 mm. in width near the aperture, distinctly outlined along its 
upper and lower margins by sharply defined revolving striations. 
Slit-band strongly convex transversely, its median parts rising 
at least ^ mm. above its lateral parts toward the aperture. Here, 
where its width is 1.5 mm., the median part of the slit-band is 
traversed by three revolving striae, equally spaced, 0.3 mm. 
apart. Of these the middle striation is traversed longitudinally 
by an extremely narrow groove, which can be detected only 
where the preservation of the shell is excellent. Along the 
periphery of the second-last volution, the slit-band is acutely 
angular instead of convex, and there is only a single revolving 
striation, but this is prominent and is located along the median 
line of the band. 
Lindstrom, Idem., pi. 11. 
Ulrich, E. O., and Scofield, W. H., The Lower Silurian Gastropoda of Minne- 
sota: Minn. Geol. and Nat. Hist. Surv., Pal. vol. 3, pt. 2, 1897, p. 957. 
