930 THE PALEONTOLOGY OF MINNESOTA. 
[Raphistomide. 
Subclass PROSOBRANCHIATA. 
Order PEHCTINIBRANCHIATA. 
Suborder KOTOMACEA. 
This proposed suborder is made up chiefly of four families of shells that are 
perhaps the most important of all to the paleontologist, namely, the Raphistomide, 
Pleurotomariide, Euomphalide and Trochide. Besides these it should include other 
families, as for instance the Missurellide and Haliotide, which were most probably 
derived from the Pleurotomariide, provisionally also the Maclureidw because of their 
evident relations to the Huomphalide. Then we include also the Turbinide because 
their early Paleozoic prototypes can be shown to have very close relations with 
unquestionable members of the group, while of their recent representatives it is 
well known that they are not greatly different from the Trochide. 
We find it most difficult to designate the characters of the shell which may 
fairly be said to be peculiarly characteristic of the group. Perhaps such characters 
do not exist,—at any rate we shall not now attempt to point them out. For the 
present it must suffice to say that the suborder rests principally upon observations 
which we regard as proving the common origin of the four families first mentioned. 
These observations will appear in the course of our remarks on the families known 
to have representatives in the Lower Silurian rocks of Minnesota. 
Family RAPHISTOMIDA, n. fam. 
This family includes shells which we regard as the best known representatives 
of the original stock from which the Huomphalide, Pleurotomariide and Trochide 
were almost simultaneously evolved. The position of the majority of the forms is 
intermediate between the first two families, leaning toward the second rather than 
the first, while the rest compare better with types that we place as early representa- 
tives of the last family, particularly with certain of the Upper Silurian shells which 
Lindstrém refers to the recent genus T’rochus. 
The most persistent character of the Kaphistomide is one that at first may seem 
almost trivial, but because of its persistency it is justly entitled to rank as 
important. Namely, the lines of growth on the upper side of the whorls, which of 
course correspond in direction with the outline of the upper lip of the aperture, 
though directed on the whole backward, are curved sigmoidally, thus causing a 
usually very slight sinus in the outer part. The curvature is never strong and is 
perhaps best developed in Raphistoma in which the point at which the change in 
