1026 © THE PALEONTOLOGY OF MINNESOTA. 
[Euomphalide. 
Pleuronotus therefore is really nothing more than a continuation of the original 
type. Such a view, however, is rendered untenable, so far as negative evidence can 
do so, by the fact that we have no knowledge whatever of the presence of the Ophileta 
type in the rocks lying between the base of the Trenton and the top of the Upper 
Silurian. Plewronotus appears to have diverged rather suddenly during early 
Devonian times from the true Huomphalus line, but we do not now feel justified 
in designating the particular species which gave it origin. As to the length of time 
that it existed, we are inclined to believe that it became extinct before the close of 
the Devonian. 
Following the development of Huomphalus into the Carboniferous rocks we find 
three types of shells: one, including EH. suwbrugosus M. and W., E. subquadratus M. and 
W., and others, in which the spire is concave, the volutions quadrangular in section, 
and the upper and lower boundaries of the broad, nearly vertical and flat or gently 
convex periphery are marked by more or less sharp angles over which the lines of 
growth pass without being much recurved. On the peripheral side these lines 
are straighter than usual, and sometimes even curved very slightly backward. This 
group reminds one greatly of the Jurassic Discohelix, and probably is to be viewed as 
the stock from which that genus sprang. The second group includes the original 
types of the genus, EH. pentangulatus and LE. catillus of Sowerby, and a number of 
other European species whose volutions have an upper and usually also a lower keel 
or angulation, with the periphery, on which the growth lines are more or less bowed 
forward, strongly convex, and the spire flat or concave. A slight sinus in the upper 
lip is common, In its typical expression this group is unknown to us in American 
deposits save through a single small species from the Upper Carboniferous of 
Missouri, which seems to be new. The third group on the other hand is well repre- 
sented here, we being acquainted with, besides several undescribed forms, the 
following five species: JL. latus Hall, Burlington gr., E. similis Meek and Worthen, 
St. Louis gr., H. planidorsatus M.and W., and E. subwmbilicatus Worthen, Chester gr., 
and EH. wmbilicatus M. and W., Coal Meas. Im all these shells the spire rises above 
the plane of the last volution, and in some of them to an unusual extent. The whorls 
are rounded on the outer and sometimes also on the lower side, but generally the 
boundary of the umbilicus is angular. The upper keel is always present and situated 
much nearer the periphery than the suture; between the latter and the keel the 
surface is flat or gently concave. The general aspect of the shells is greatly like 
that of the prevailing forms of the Lower Silurian genus Helicotoma. In some this 
resemblance extends even to the possession of a number of obscure revolving lines on 
the peripheral region like those seen in H. planulata. There is, however, a very 
decided difference in the course of the lines of growth marking the surface of the outer 
