NO. 3 UPPER CAMBRIAN TRILOBITE FAUNAS — RASETTI 37 



may be extremely similar (e.g., in Tennessee the early species A. 

 walcotti resembles the late species A. tarda), not much time signifi- 

 cance can be attributed to such similarities. In Nevada, Palmer finds 

 Tumicephalus (based on a species almost identical with Aphelaspis 

 tumifrons) in the ^ost- Aphelaspis, Dicanthopyge zone, species of 

 Dytremacephalus in the still younger Prehousia and Dunderbergia 

 zones, Cheilocephalus hrachyops and various species of Dunderbergia 

 in the Dunderbergia zone. These findings might be taken to indicate 

 that part of the middle Aphelaspis zone and all of the upper Aphelaspis 

 zone of this paper are equivalent to portions of Palmer's post-Aphel- 

 aspis zones in Nevada. On the other hand, these latter zones are char- 

 acterized by a large number of genera of Pterocephaliid trilobites 

 never observed in Tennessee. Hence it is difficult to decide what fauna 

 in Nevada is the equivalent of the youngest Aphelaspis faunule from 

 Tennessee, the Aphelaspis tarda faunule, which occurs in the upper- 

 most beds of the Nolichucky formation. Perhaps the evidence favors 

 equivalence to part of the Prehousia or Dunderbergia zones in Nevada. 



Even in the lower part of the Aphelaspis zone several genera present 

 in the early Aphelaspis fauna in Alabama and Nevada were not found 

 in Tennessee. Conspicuous among these are Glyptagnostus and other 

 agnostid genera, and the Pterocephaliid genus Olenaspella, indistin- 

 guishable from Aphelaspis in the cranidial features but characterized 

 by the development of one or more pairs of marginal pygidial spines. 

 Apparently these trilobites are more characteristic of the geosynclinal 

 facies and never migrated far enough within the continental shelf to 

 reach northeastern Tennessee. Absence of trilobites of the geosynclinal 

 facies is a feature of both the Crepicephalus and Aphelaspis faunas. 



The observations in Tennessee bear no evidence on the question 

 whether the replacement of the Crepicephalus fauna with the Aphel- 

 aspis fauna was simultaneous or not in different areas, since the locali- 

 ties studied are all within a narrow belt parallel to the margin of the 

 geosyncline. 



In Tennessee no occurrence of Cedaria or other genera of the 

 Cedaria zone close to the base of the Aphelaspis zone, or even a few 

 hundred feet below this level, was ever observed. This fact may again 

 be related to the "cratonic" rather than geosynclinal environment pre- 

 vailing through the entire Upper Cambrian in Tennessee. 



