950 THE PALEONTOLOGY OF MINNESOTA. 



[Pluerotomarildm . 



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 lunulae 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 lunulse 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 

 Lindstrom sp., and our L. notabilis, the lunulae 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 (Pleurotomaria limata Lindstrom), or they are crossed by one (PL 

 ohioensis James) or two (PL scutulata Lindstrom) median lines; or the central line 

 may be developed into a thin undulating plate (Lophospira sermlata Salter sp.) or 

 into a row of nodes (PL [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 



