1006 THE PALEONTOLOGY OF MINNESOTA. 
Clathrospira subconica. 
modification is in accordance with a tendency that has been followed almost 
generally in the development of the whole family. As a rule it appears to have 
preceded the development of the apertural slit. 
At present we regard Clathrospira as the stock from which the Devonian group 
of species that includes Pleurotomaria lucina, P. hebe, P. itys, P. filitexta and P. ella 
(all described by Hall) was derived. The aperture in all of these species, so far as 
known, is without a slit, and the surface cancellated. Taken as a group, their 
whorls are more rounded than in their supposed Lower Silurian ancestors, but this 
difference, provided that our information concerning the remaining characters is 
reliable, would be no serious objection to classing them as congeneric with Clathro- 
spira subconica. Still, we cannot be too cautious in matters of this kind, and we 
wish to be understood as merely suggesting and not as proposing the transference 
of these Devonian species to Clathrospira. If the old genus Pleurotomaria is to be 
successfully broken up according to the principles of ontogenetic classification, we 
must be reasonably sure of our ground before making sweeping changes in nomen- 
clature. For other remarks on these Devonian shells see page 956. 
CLATHROSPIRA suBCONICA Hall. 
PLATE LXIX, FIGS. 47—50; PLATE LXX, FIGS. 5—6 
Pleurotomaria subconica HALL, 1847, Pal. New York, vol, i, pp. 174 and 304; WHITFIELD, 1883, Geol. 
of Wis., vol. iv, p. 216. » 
Shell with a short conical spire, consisting, when fully grown, of six and a half or seven volutions, 
of which the two at the apex are usually broken away; greatest width and hight nearly equal, varying 
generally between 25 and 30 mm., but attaining occasionally a width of over 40 mm.; apical angle 
70° to 80°, but in four specimens out of every five the variation is only about one degree either 
way from 74°. Volutions flattened above in the direction of the slope of the spire, the inner 
half of the slope gently convex, the outer half correspondingly, or more strongly concave; convex 
portion of slope just touching or failing to reach a line drawn from periphery to periphery of 
succeeding whorls; under side of whorls rounded, occasionally very slightly concave near the periphery, 
this condition appearing, however, only in specimens in which the band is unusually prominent, 
umbilical depression small, terminating generally in a minute axial perforation. Band prominent, 
sharply defined, rather wide, concave, nearly vertical, situated on the periphery of the last volution, and 
lying immediately above the suture line on the upper whorls. Aperture subquadrate, outer lip broadly 
notched; columellar lip not very strong, thin, folding about the small umbilical perforation. Surface 
sculpture beautifully cancellated, consisting of two sets of fine, subequal, thread-like lines, one revolving, 
the other running parallel with the margin of the aperture. The transverse lines, of which three to five 
occur in the space of 1 mm., are recurved as usual on the upper side and quite as much on the lower side. 
At intervals, sometimes quite regular, many specimens exhibit more or less distinct undulations of 
growth, in some examples little more than a millimeter apart, in others two, three, or four millimeters. 
Considerable variety as regards strength and arrangement of the lines forming the surface sculpture may 
be observed in specimens from different localities. Asarule the revolving lines are strongest on the basal 
portions of the shell, and in some of the specimens from the ‘‘Glade limestone” of Tennessee they appear 
to be wanting entirely on the upper side. In the latter cases the transverse lines are stronger than usual 
In all the specimens from the Stones River group of Illinois, Wisconsin and Minnesota that retain the 
markings, the two sets of lines are almost equally developed, and on the whole finer than on the Tennessee 
