994 THE PALEONTOLOGY OF MINNESOTA. 
{Liospira micula. 
tomaria numeria Billings (Quebec group), Pl. eugenia Billings, Helicotoma larvata 
Salter (Black River group), Pl. subtilistriata Hall (Trenton group), and Pl. 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 EHotomaria. 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. rugata, L. mundula and L. angulata) 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. helena 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. 
Liosprra micuLta Hall. 
PLATE LXVIII, FIGS. 24—29. 
Pleurotomaria micula HALL, 1862, Geol. Rep. Wis., p. 55. (Figured but not described.) 
A small discoidal shell 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 slightdy 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 make a broad curve, the inner half being the most curved. The band occupies the periphery 
