STRATIGRAPHIC TAXONOMY 615 



from the "Lower Magnesian" farther north and those collected from 

 what is now known as the Hoyt limestone near Saratoga, New York, 

 and described by Walcott, most of which were recognized in a middle 

 division of the Missouri section, this large fauna is entirely new to 

 science. Viewing only the trilobites, it must be called a typical develop- 

 ment of the Dikellocephalus fauna; and if by that designation we refer 

 to the typical Saratogan development of crustacean life, the appellation 

 is fully Justified when applied to this Missouri facies. Though Dikello- 

 cephalus-like trilobites occur in the underlying upper Cambrian beds in 

 Missouri as elsewhere, it yet seems a fact that the typical D. minneso- 

 tensis section of the genus is confined to the Ozarkian part of the time 

 scale. Beneath its first occurrence in the latter there is in Missouri at 

 least one formation — the Potosi — and in Alabama three formations aggre- 

 gating over 2,000 feet of dolomitic beds, in which neither type of the 

 genus has been observed. ]^or is the upper Cambrian type of the genus 

 k:nown to occur above this unfossiliferous interval. Ample time, there- 

 fore, intervened between the two occurrences to render it reasonably 

 certain that the presence of D. minnesotensis or of closely affiliated species 

 is diagnostic of the Ozarkian period. And there are other derivatives 

 of Cambrian trilobites which, whether viewed as generically or but 

 specifically modified, appear to be no less characteristic of the Ozarkian. 



But it is the advent of a host of gastropods and cephalopods, of types 

 entirely unknown in true Cambrian rocks, that stamps the Ozarkian as 

 a new period in geologic history. Small coiled shells, with depressed 

 spires and but one or two whorls — probably heteropods which have been 

 wrongly associated with the genus Platyceras — are found occasionally 

 in the Cambrian and up to the middle of the Ozarkian. Patelloid gas- 

 tropods also are found in the Cambrian and apparently maintained an 

 unbroken line to the present day. But high-spired PleurotomariidcB , 

 likewise low-spiried Liospira-like types of this family, also well developed 

 Raphistomidce, Euomphalidce, and Holopea-like forms, these are seen 

 for the first time in the Ozarkian. The cephalopods, though obviously 

 of primitive types, are yet readily comparable in average size and charac- 

 ter with their Canadian and Ordovician descendants. They first become 

 abundant somewhat later than the gastropods, but both classes had at- 

 tained fair development in Missouri before the typical Saratogan trilobite 

 fauna became established there ; and they reached the highest develop- 

 ment recorded in the period before Dil-ellocephalus passed out of ex- 

 istence. 



The sequence and aggregate thickness of deposits in the Appalachian 

 Vallev referred to the new Ozarkian svstem have been set forth at 



