948 THE PALEONTOLOGY OF MINNESOTA. 



[PleurotomariidtB . 



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 

 Pleurotomariidce 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. Lindstrom 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 be a 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 

 Pleurotomariidce, 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 Bicinda 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 Bicinda section, so that the suggested 

 explanation of the development of the slit cannot apply here and is rendered highly 

 improbable in any case. 



