J. IMBRIE 



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fossil record of an evolving succession of populations. Four tran- 

 sient species belonging to one lineage are symbolized by numbers 

 1 through 4. Under ideal conditions, fossils representing the en- 

 tire lineage could be studied in outcrops of the appropriate facies. 

 In practice, however, the geological column can be studied only 

 in isolated, accessible areas. Hence a paleontologist examining 

 fossils from region A records two transient species 1 and 3, while 

 in region B his observations are limited to transients 2 and 4. If 



TALLAHATTA LOWER WECHES UPPER WECHES 



Fig. 13. Some of the characteristic features of four species of the 

 Eocene oyster Cubitostrea. Heavy stippling represents auricles. Formation 

 names indicate the stratigraphic position of individual species, as follows: 

 Tallahatta, C. perplicata; Lower Weehes, C. lisbonensis; Upper Weches, C. 

 smithvillensis; Cook Mountain, C. sellaeformis (Stenzel, 1949). 



morphological gaps between studied transients are too large, 

 sound phylogenetic interpretations may be impossible. 



One of the best documented examples of transient species re- 

 cording portions of an evolving lineage is Stenzel's ( 1949 ) study 

 of the oyster Cubitostrea. Four clear-cut species have been recog- 

 nized, each restricted to a different stratigraphic level in the Eo- 

 cene of the Gulf Coast. C. perplicata, the oldest known species in 

 this line, is characterized by small size, strong ribbing, thin shell, 

 absence of auricles, lack of arching, as well as other characters 



