196 The American Geologist. April, isoo 
consequently exceedingly broad and shallow. The whorls are 
regularly rounded especially in the younger examples, but the 
larger individuals often show a decided tendency to become 
subangular. In most of the specimens the turns are barely in 
contact and in many cases even separated towards the aper- 
ture so that there is, especially in internal casts, a striking 
resemblance to those evolute planorbicular forms for which 
Sowerby proposed the term Fhanerotinus. 
The two remaining forms to be mentioned are both from the 
beds of the Burlington limestone — the one occurring only in 
the lower part, and the other only in the upper. The first is 
S. latus (Hall) [figs, la, lb] and is by far the largest species 
known from the locality, frequently having a diametric meas- 
urement of more than eight centimeters. The whorls above 
are nearly on a level and are sharpl}^ angular towards the 
sutural line and also just above the periphery, making the 
upper surface transversely flat or slightly concave. The 
carinae are manifestly thickenings in the shell as exhibited in a 
cross section [fig. lb] and as also shown by the regularly 
rounded natural casts of the interior of the shell. The volu- 
tions may also have an obtuse angularity below. 
In S. roherti (White) [figs. 3a, obj the apical parts are 
depressed below the upper surface of the outer whorl, thus 
giving additional prominence to the carinae ; while the umbil- 
icus is correspondingly more shallow. 
It has been suggested that the latter form was the genetic 
successor of S. latus, and this is probably the case since the 
stratigraphy of the locality indicates that the various layers 
were laid down under practically the same physical conditions ; 
and the testimony furnished by the exceedingly rich and 
varied fauna of the area amply corroborates the same conclu- 
sion. This would rather point to the assumption that the 
faunse of the so-called Upper and Lower divisions of the Bur- 
lington limestone, and perhaps also even of the Keokuk in 
the immediate neighborhood, were closely related biologically 
and that they were not merely occupants migrating to the 
district at different times from divers localities which belonged 
to faunal regions having no actual relations. 
EXPLANATION OF PLATE. 
Fig. 1. Straparollus latus (Hall), la. View of an average specimen 
