950 THE PALEONTOLOGY OF MINNESOTA. 
[Pluerotomariide. 
What we conceive to be the most primitive type of slit-band occurs in the 
Lower Silurian Lophospira. In this genus it occupies, or rather forms, the summit 
of a more or less prominent peripheral ridge, above and beneath which the lines of 
growth curve backward in directions corresponding to the outline of the apertural 
notch. The band itself may be simply a blunt edge, upon which the growth lines 
make their turn; but more commonly it is defined on each side by a delicate raised 
line, which separates the lunule of the band from the surface striations. The 
markings of the band are always different from that of the rest of the surface, 
being as a rule more regular, while the arched transverse lines or lunules may be 
stronger or weaker, and farther apart or closer than the lines of growth either above 
or beneath the band. Occasionally, as in Lophospira tubulosa and L. imbricata 
Lindstrém sp., and our L. notabilis, the lunule are widely separated and strongly 
imbricating; sometimes, as in L. bicincta, they are much finer and very closely 
arranged; frequently they consist of simple elevated lines; in other cases they have 
a median excision (Plewrotomaria limata Lindstrém), or they are crossed by one (PI. 
ohioensis James) or two (Pl. scutulata Lindstrém) median lines; or the central line 
may be developed into a thin undulating plate (Lophospira serrulata Salter sp.) or 
into a row of nodes (PI. [Worthenia] tabulata Conrad). In short, the marking of 
the band in the members of the family is of great variety, but as a rule we cannot 
say that the various types are of much assistance in determining the generic or 
subgeneric position of the species. 
Although we can already see some possible exceptions, we think that provis- 
ionally it is advisable to regard species having a concave band as generically 
distinct from those in which the band is convex. We have very carefully examined 
a large number of species, and so far the separation on this difference has resulted 
in a very much more satisfactory classification than any we have yet had. Consid- 
ering the form of the band, as far as its being concave or flat on the one hand and 
more or less convex on the other is concerned, as a leading test of relationship, we 
bring together many forms that have hitherto been separated, while many others 
that had been associated are widely separated. 
As generally understood heretofore, particularly among American and Canadian 
paleontologists, the two principal Paleozoic genera of the family are Pleurotomaria, 
De France, and Murchisonia, d@Archiac and Verneuil, the former embracing the 
species with a low spire and relatively few volutions, the latter those forming a 
high shell of numerous whorls. As viewed by us this broad separation or arrange- 
ment of the species according to the hight of the spire results in a most artificial 
classification, since it causes the separation of shells that comparative studies prove 
to be closely related genetically, while others are associated that have only very 
