34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 148 



collected. Two examples, the presence of Aphelaspis cf. lata at an un- 

 usually high horizon in collection cnr/4, and the early occurrence of 

 Dytremacephalus angulatus in collection cnt/4, probably indicate that 

 while at a given locality each species was abundant for only a very 

 limited time, it may have lived at different times at other places. The 

 inversion of the order of the Aphelaspis camiro and Aphelaspis rotun- 

 dafa assemblages, observed in two sections, even though it does not 

 involve a considerable thickness of strata, is another indication that 

 the presence or absence of a species at a certain level may have little 

 time significance, being determined rather by some ecologic factor, or 

 simply a historical accident. 



Even considering these restrictions to its validity, it is clear that 

 the above-discussed succession of Aphelaspis faunules is the best docu- 

 mented in the entire Cambrian of the Appalachians and may allow the 

 stratigrapher to correlate new sections with great precision. As it will 

 be pointed out in the discussion of Aphelaspis, a species of that genus 

 may not be identifiable from a single individual, even if perfectly pre- 

 served, much less from weathered or flattened material. Fortunately, 

 however, the strata of the Aphelaspis zone, and especially thin, crystal- 

 line limestone lenses in shale, may be so fossiliferous that a small 

 sample of rock in many cases supplies sufficient information for the 

 clear recognition of one of the faunules described herein. 



In view of the relatively meager knowledge of the biostratigraphy of 

 the Aphelaspis zone in North America and of the observed transition 

 between the Crepicephalus and Aphelaspis faunas, a comparison with 

 recent findings in other areas is of interest. A detailed study of the 

 distribution of genera and species of trilobites near and above the 

 Crepicephalus-Aphelaspis zone boundary was made by Palmer ( 1954, 

 1962b) in Alabama, central Texas, and Nevada. His main conclusions, 

 derived from the study of the faunal succession in the Conasauga 

 formation at Woodstock and Cedar Bluff, Ala., and in the uppermost 

 part of the Hamburg limestone and the overlying Dunderberg shale at 

 several localities in Nevada, may be thus summarized : ( 1 ) There is a 

 sudden change, often occurring in a few feet of strata, from a faunule 

 containing trilobite genera of the Crepicephalus zone ("Crepicephalid 

 biofacies") to a faunule with Aphelaspis and other trilobites of the 

 Aphelaspis zone ("Pterocephaliid biofacies"). (2) This change in Ne- 

 vada coincides with the contact between the relatively pure, massive 

 Hamburg limestone and the interstratified siltstone and silty limestone 

 beds of the overlying Dunderberg formation. (3) Notwithstanding 

 the great faunal change, no appreciable hiatus is believed to be in- 

 volved. (4) The replacement of the Crepicephalid biofacies with the 

 Pterocephaliid biofacies is believed not to have been synchronous every- 



