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 



Pkurotomaria 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 bight 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. As a rule 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 Elver 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 Tenn essee 



