912 THE PALEONTOLOGY OF MINNESOTA. 
(Oxydiscus. 
The chief peculiarity of C. imbricata lies in the fact that the sides of the whorls 
are the most prominent and somewhat angularly bent just beneath the middle and 
flattened in the umbilicus, giving a subrhomboidal or Cyrtolites-like transverse 
section instead of the rounded or cordiform section prevailing among the other 
species of the genus. In all other respects the species agrees very well with C. 
triangularis U. & 8. and C. elegans Miller. Still, the transverse lamelle are more 
irregularly undulating and wider apart than in either of those forms. 
Meek and Worthen make a statement to which we must object. Namely, that 
the inner volutions are “nearly half embraced by the last turn.” This is true of 
only the anterior part of the last volution of their type, and we are inclined to 
believe that it is due to oblique pressure to which the specimen has been subjected. 
On the rest of the type, as in the whole of the specimen now illustrated, the 
embracing extends only to the base of the carina. 
Formation and locality.—In strata regarded as belonging to the Trenton group, Alexander county, 
Illinois. The same layers contain Rhynchotrema incequivalvis Castelnau (R. increbescens Hall) and Nemato- 
pora delicatula Ulrich. 
Collections.—Illinois State Museum; E O. Ulrich. 
Genus OX YDISCUS, Koken. 
Bellerophon, Cyrtolites, Porcellia and Euomphalus (part.) of authors. 
Tropidodiscus of MEEK, 1866, and WAAGEN, 1880, not Steininger, 1855. 
Oxydiscus, KOKEN, 1889, N. Jahrb. f. Mineralogie, etc., Beilageband vi, p. 390. 
For generic characters see page 852. 
This group recommends itself to us as not only a convenient but a natural 
generic division of the Bellerophontacea. Sowerby, Billings and Conrad placed three 
of the species under Bellerophon, from which they are distinguished by their lenticular 
form, compressed and sharply carinate volutions, scarcely, if at all, expanded 
aperture, and by their thin lips and the total absence of any callosity. Lindstrom 
and Miller placed four of the species with Cyrtolites, a genus that is widely different 
in all respects excepting the general form. As to Porcellia, Koken has shown, and 
we can bear evidence for the general correctness of his observations, that that 
genus represents a totally different type of structure. Finally, the reference of two 
of the species to Euomphalus rests probably upon nothing more than an error of 
observation and judgment. 
Koken excludes all those species from the genus in which even a suspicion of a 
slit-band occurs. This we think is drawing the line too close and somewhat incon- 
sequent. He admits into Bellerophon (sens. strict.) species in which the slit-band is 
represented by a single keel only. And he is probably correct in this, since in not 
only closely related species but in one and the same species (e. g., B. troosti Safford) 
a true slit-band may occur occasionally, while the usual form has merely a keel. 
