MEDINAN, NIAGARAN, AND CHESTER FOSSILS 
97 
In the last volution, the vertical outline above the slit-band 
is gently concave, the concave curvature being more pronounced 
toward the band. Below the slit-band the vertical outline 
is convex, with a tendency toward a concave outline within 2 
mm. of the band, due chiefly to the prominence of the peripheral 
portion of the shell bearing this band. In the second-last volu- 
tion, the slope of the shell above the slit-band is interrupted at 
mid-height by a very prominent revolving striation, dividing 
this slope into two equally concave revolving areas. Along the 
last volution this striation becomes less prominent and occupies 
a position increasingly farther up the slope, being three-fifths 
of the distance from the slit-band to the suture above, near the 
aperture. Along this part of the shell the striation in question 
is merely slightly more prominent than those above and below, 
and does not interrupt conspicuously the general concave out- 
line between the slit-band and the suture above. Above this 
more prominent striation, the surface of the last volution is 
marked by 9 revolving striae. Beneath this more prominent 
striation there are 4 striae about as prominent as those above 
this striation, and then a fifth striation distinctly more promi- 
nent than any of the preceding four is found. The upper half 
of the space between the fifth striation and the slit-band is 
occupied by 5 much fainter striae, with a sixth faintly visible 
a short distance beneath. Along the second-last volution only 
4 or 5 revolving striae occupy each of the two concave areas 
forming the visible part of this volution between the slit-band 
and the suture above. 
Immediately below the slit-band, along the concave part of 
the shell, toward its aperture, the revolving striae are very faint 
toward the band but become increasingly stronger at a greater 
distance. At least 40 revolving striae occur between the slit- 
band and the umbilical portion of the shell. These tend to be 
approximately equal in size, though slight alternation in promi- 
nence is noted. In general the prominence of these striae 
distinctly exceeds that of most of the striae along the upper 
two-thirds of that part of the last volution which is above the 
slit-band. 
