668 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



gray, fine grained dolomite and contains the only known occurrence of 

 the Phyllograptns fauna in American interior rocks. With the grapto- 

 lites, of which the following have been determined, Phyllograptus illi- 

 cifolius, P. angustifolius, Didymograptus hifidus, and D. amplus, occur 

 two possibly undescribed orthoid brachiopods (related to Dalmanella 

 wemplei) and fragments of two trilobites, one an Asaphus akin to A. 

 canalis. Over this comes first a 6 foot bed filled with drusy and solid 

 hard chert containing numerous fossils, among them Plethospira cassina, 

 SubuUtes olesus, and Eurystomites hellogi, all of which were described 

 from the Cassin limestone of the Champlain Valley. It is probably this 

 bed that is developed to a thickness of nearly 20 feet in the valley of 

 Swan Creek, in Christian County, Missouri, and from which a well pre- 

 served fauna of more than 20 species has been collected. Near Smith- 

 ville it is followed by a 1 to 4 foot bed of gray, crystalline, magnesian 

 limestone, containing the slender noncarinate form of the opercula for 

 which the new generic name Ceratopea is proposed on page 665. At 

 other points in the vicinity of Smithville this bed either expands to 10 

 and even 20 feet, or another thin bed of limestone succeeds it. The 

 latter is filled with a large and well preserved moUuscan fauna. The 

 fossils occur silicified in porous chert or free in the residual clay. Some 

 of them are identical with well known Cassin species, while others seem 

 closely allied to species occurring locally at the top of the Shakopee in 

 Minnesota and Wisconsin, and to others found in the middle part of 

 the Arbuckle limestone in Oklahoma. This bed is usually succeeded in 

 Lawrence County by a secondarily silicified chert conglomerate, a few 

 inches to several feet thick, and this by loose quartz sand probably de- 

 rived from a decomposed calcareous sandstone referable to the Everton. 

 Locally, however, as in the quarry at Black Eock, the hiatus at the top 

 of the Ceratopea bed is partially represented by about 40 feet of piu'e 

 and argillaceous limestone, most of it filled with undescribed sponges 

 and bryozoa. The fauna includes also brachiopods, of types found in 

 preceding beds, and graptolites like Dendrograptus flexuosus and a small 

 variety of Didymograptus hifidus. But the mollusks which are so con- 

 spicuous in the underlying members are almost entirely absent. 



Nearly everywhere in northern Arkansas and southern Missouri the 

 upper part of the Jefferson City dolomite, the top formation of the 

 Ozarkian, contains the hemispherical colonies of Cryptozoon minneso- 

 tense in abundance. This excellent guide fossil, therefore, has proved 

 of great service in locating the boundary between the Jefferson City and 

 Yellville formations and hence between the two systems — Ozarkian and 



