1026 THE PALEONTOLOGY OP MINNESOTA. 



[Euomphalidae. 



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. Pleuronotus appears to have diverged rather suddenly during early 

 Devonian times from the true Euomphalus 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 Euomphalus into the Carboniferous rocks we find 

 three types of shells: one, including E. subrugosus 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, E. pentangulatus and E. 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: E. latus Hall, Burlington gr., E. similis Meek and Worthen, 

 St, Louis gr., E. planidorsatus M. and W., and E. subumbilicatus Worthen, Chester gr., 

 and E. umbilicatus M. and W., Coal Meas. In 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 



