846 THE PALEONTOLOGY OF MINNESOTA. 
[Cyrtolitide. 
and Patellacea in the same order, we have taken the probable, yet not established, 
connection between Stenotheca and Cyrtolites into consideration, our principal reason 
for doing so lies in the fact that in both the shell is symmetrical. 
During the preparation for this work we undertook as nearly a complete 
revision of the American species of the suborder as possible, the cabinet of one of 
the authors containing representatives of a large proportion of the described forms. 
Much time was spent also on the foreign species, though here we were obliged to 
rely almost entirely upon the published figures. Still, with our knowledge of the 
American species, the illustrations alone. were in most cases sufficient for our 
purposes. Our comparative studies, though demonstrating the close interrelation- 
ship of the various types comprised in the suborder, still showed very clearly the 
actual existence of numerous natural and in most cases easily recognized groups of 
species which we deem of sufficient importance to rank as genera. Most of these 
have been already established, but, as some require sharper limitation and correction, 
we shall endeavor to characterize them all as fully and clearly as we can now do. 
Under each genus we add a list of the American species, and when desirable of 
the foreign as well, which we have examined and of whose affinities and position 
we are reasonably certain. 
We recognize twenty-three genera and divide them into five families. In 
the following synopsis brief discussions follow the diagnosis of the genera of which 
no species are described by us, while the remarks on the others will be found 
immediately preceding the descriptions of the species of each. 
Family CYRTOLITIDA. 
Symmetrical involute shells; volutions two or three, barely in contact, sharply 
angular dorsally; aperture not expanded, the sinus V-shaped, never deep, sometimes 
wanting; no slit, and the band occurs only in Cyrtolitina; surface reticulate. 
Cyrtrouites, Conrad. Shell coiled in the same plane, symmetrically or nearly so; 
volutions two or three, scarcely contiguous, the last occasionally free, enlarging 
gradually, carinated on the back and often on the sides, giving a subquadrate cross- 
section; aperture not abruptly expanding, with or without a median notch in the 
outer lip; no slit-band; shell thin, without callosities of any kind; surface sculpture 
reticulated or cancellated, consisting of straight or obliquely curved regular 
transverse lines connected by short oblique lines. ‘Type, C. ornatus Conrad. 
C. ornatus Conrad. Cincinnati period. C. subplana Ulrich. Trenton group. 
C. ornatus, var. minor U. & 8. Trenton group. QO. parvus Ulrich. Utica group. 
C. retrorsus Ulrich. Black River, Trenton,and Utica C. carinatus Miller. Utica group. 
groups. QC. disjuncius U. & 8. Richmond group. 
C. retrorsus, var. fillmorensis U. & S. Black Riv.gr. (C.? dilatus U.&S. Black River group. 
