912 THE PALEONTOLOGY OF MINNESOTA. 



[Oxydiseus. 



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. & S. and C. elegans Miller. Still, the transverse lamellae 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, 

 I Hindis. The same layers contain Hhynchotrema incequivalvis Castelnau (li. increbcscens Hall) and Nemato- 

 pora delicatula Ulrich. 



Collections. Illinois State Museum; E O. Ulrich. 



Genus OXYDISCUS, Koken. 



Bellerophon, Cyrtolitea, Porcellia and Euomphalus (part.) of authors. 

 Tropidodiscus of MEEK, 1866, and WAAGEN, 1880, not Steininger, 1855. 

 Oxydiscus, KOKEK, 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. 



