948 THE PALEONTOLOGY OF MINNESOTA. 
[Pleurotomariide. 
the slit-band, each into two similar groups, one having the band convex, the other 
concave. Such an arrangement might at first appear convenient, yet a careful 
study will soon reveal that it would be quite arbitrary, therefore unnatural, and in 
the end not even convenient. This is so obvious that it is quite unnecessary to cite 
proving instances. Still, there is an element of truth in the first of these suggested 
divisions, for it would separate what we may call the archaic from the more recent 
stages in the development of the family. 
We have already referred to the almost total absence in the Lower Silurian 
Pleurotomariide of the long parallel-edged slit which occurs so generally among the 
more recent types of the family. This difference has not received the attention 
from paleontologists that it deserves, for surely it must indicate a structural 
difference in the animals. Lindstrém barely alludes to it, while Koken, much to our 
surprise, takes no notice of it whatever. Unfortunately, the presence or absence of 
the slit is not positively determinable except when the aperture is entirely preserved, 
the band behind the slit presenting, so far as we can say, no evidence that might 
lead the observer to suspect either the one or the other condition. 
The slit which should be carefully distinguished from the apertural notch, which 
is more or less widely V-shaped and does not extend backward any farther than the 
bottom of the sinus formed by the lines of growth, seems to bea later phase in the 
evolution of the majority of the lines of development that can be traced from the 
Lower Silurian into subsequent periods. Its development appears to be the result 
of a tendency to which the whole family, rather than any particular generic line, is 
subject. As we have already said it is almost entirely absent in the Lower Silurian 
Pleurctomariide, in the majority of which the lines of growth, and, therefore, the 
outer edges of the lip, sweep backward toward the band more strongly than in the 
prevailing types of subsequent ages. We might then assume that the slit was 
represented in those ancient times by a deep notch, and that the presence of the 
latter in many Devonian and Carboniferous forms is merely a retention of a primor- 
dial character after its real cause or purpose had been removed or satisfied by the 
development of a long slit. However, before such a view can be accepted we must 
account for the extreme shallowness of the apertural sinus in such slit-less species 
as are comprised in the Bicincta section of Lophospira, which, as far as our present 
knowledge goes, existed in numerous species from the Chazy to the close of the 
Upper Silurian. Now, Shizolopha textilis, which has a long slit, was almost 
certainly developed from some member of the Bicincta section, so that the suggested 
explanation of the development of the slit cannot apply here and is rendered highly 
improbable in any case. 
