NO. 3 UPPER CAMBRIAN TRILOBITE FAUNAS RASETTI 35 



where, but to have taken place earlier in the geosynclinal areas and to 

 have spread later toward the interior of the continental shelf. Hence 

 the use of the term "biofacies" rather than faunizones. (5) The upper- 

 most faunule attributed to the Crepicephalid biofacies at McGill, Nev,, 

 and a similar faunule collected at Woodstock, Ala., include Cedaria 

 prolifica, the type species, or related forms of Cedaria. If the inter- 

 pretation is correct, the type species of Cedaria would be a trilobite of 

 the Crepicephalus rather than the Cedaria zone. 



In a second, more extensive paper on the Upper Cambrian faunas 

 of Nevada, Palmer (1965)^ recognizes in ascending order the Aphel- 

 aspis, Dicanthopyge, Prehonsia, Dunderhergia, and Elvinia zones, all 

 belonging to a Pterocephaliid "biomere." The vahdity of some of these 

 zones may be limited to the Great Basin. 



The observations in Tennessee show several features in common 

 with the faunal development in Nevada. The change from the Crepi- 

 cephalus to the Aphelaspis fauna is equally sudden, and the writer has 

 already expressed agreement with Palmer's conclusion that most of 

 the trilobites of the Aphelaspis fauna have no immediate ancestors in 

 the Crepicephalus fauna ; hence the change occurred by immigration of 

 a new fauna that had been evolving elsewhere, presumably in geosyn- 

 clinal areas. In Tennessee the faunal change may occur either within 

 the lower, massive limestone member of the Nolichucky formation, or 

 coincide with the contact between this limestone unit and the over- 

 lying shale with interbedded, thin limestone beds and lenses ; or it may 

 occur within the latter lithologic unit. In any case there is no evidence 

 whatever that the faunal change is associated with a hiatus. 



The earliest Aphelaspis faunule observed in Tennessee has species 

 identical with or closely related to some of the earliest species of the 

 Pterocephaliid biofacies found by Palmer in Nevada. In particular, 

 Aphelaspis buttsi is the earliest species of the genus appearing both at 

 McGill, Nev., and in the Three Springs section in Tennessee. Instead, 

 no very close relationship appears between the youngest faunules of 

 the Crepicephalid biofacies observed in Nevada and Tennessee; only 

 the genus Coosia seems common to both. This fact might be indicative 

 of the presence of a hiatus in the Nevada sections. 



It becomes more difficult to correlate younger faunules from Ten- 

 nessee with those of the Nevada sections. Several Aphelaspis species 

 described from Nevada, A. brachyphasis, haguei, subditus, and longis- 

 pina, all closely resemble species from Tennessee, but in view of the 

 fact that in either region species of Aphelaspis of rather different ages 



^ The writer is indebted to Dr. Palmer for reading the manuscript before pub- 

 lication. 



