994 THE PALEONTOLOGY OF MINNESOTA. 



[Liosplra nilcula. 



tomaria numeria Billings (Quebec group), PI. eugenia Billings, Helicotoma larvata 

 Salter (Black River group), PL subtilistriata Hall (Trenton group), and PI. helena 

 Billings (Hudson River group) as belonging here. Then there is a considerable 

 number of other Lower Silurian lenticular shells which cannot be placed satisfacto- 

 rily because the most essential characters are neither mentioned in the descriptions 

 nor shown in the figures so far published. There may be some of Liospira among 

 them, but there is just as much reason for referring them to Raphistoma, Raphistomina 

 or Eotomaria. Finally, there is a group of species of which Pleurotomaria docens 

 Billings is a good representative. This species was recently referred to Raphistoma 

 and it cannot be denied that in the general form of the shell it agrees very well 

 with the most typical species of that genus. Still, P. docens has a band and that 

 alone positively forbids its being referred to Raphistoma. Now, as to the band, is 

 it entirely on the upper side of the peripheral angle, or does it turn over the edge 

 and thus lie upon both sides of it? Judging from a Tennessee specimen which 

 Billings himself identified with his species, we should say that the latter is the case 

 and, therefore, that the species is to be viewed as a flat-topped Liospira. 



As at present constituted Liospira includes five species (L. larvata Salter sp., L. 

 numeria Billings sp., and our L. ruaata, L. mundula and L. anaulata) that remind one 

 very strongly of the euomphaloid genus Helicotoma. But they all have an unques- 

 tionable slit-band and one that, so far as we have been able to learn, is in every 

 respect like that of the most typical forms of Liospira. Helicotoma, however, has no 

 true slit-band. Instead we find merely a thin sharp ridge with occasionally a line 

 on only one side of its base placed entirely upon the upper side of the whorls. 

 That these species belong to Liospira is, we believe, shown conclusively by such 

 obviously intermediate forms as L. subconcava, L. persimilis, L. heltna and L. eugenia. 

 It seems to be merely another case showing how some of the subdivisions of 

 ordinarily widely different branches of the same radical may come to agree in 

 structure through, in this case we presume, a kind of atavism. 



LIOSPIRA MICDLA Hall. 



PLATE LXVIII. FIGS. 24-29. 



Pleurotomaria micula HALL, 1862, Geol. Rep. Wis., p. 55. (Figured but not described.) 



A small discoidal 8hell having the umbilicus filled by a reflexed callosity of the inner lip. This 

 filling is concave externally, perfectly smooth, and generally rather distinctly outlined and distinguished 

 from the finely striated under side of the volutions. There are about four volutions, and within these a 

 minute nucleus, the sutures are very shallow, the spire, excepting an occasional slight convexity of the 

 upper whorls, forming an almost continuous even slope from the apex to the slightly obtuse periphery. 

 The surface markings consist usually of fine lines of growth only, but nearly all of the best specimens 

 show traces of an exceedingly fine set of revolving lines. On the under surface of the whorls the lines of 

 growth mike a broad curve, tha lunar half being the most curved. The band occupies the periphery 



