J. IMBRIE 147 



(Fig. 13). C sellaeformis, the youngest known species, is charac- 

 terized by large size, absence of ribs, thick shell, strongly devel- 

 oped auricles, and conspicuous arching. Gaps between these 

 extremes are partly bridged by C. lisbonensis and C. smithvillen- 

 sis. 



This assemblage of species is viewed as the fossil record of a 

 continuously evolving stock leading to C. sellaefonnis. But the 

 point to be emphasized here is that each of these species repre- 

 sents only a stage in this evolution and is a transient, not a suc- 

 cessional, species. The stratigraphic range of each species is indi- 

 cated in Fig. 14. Although Cubitostrea is represented almost 

 continuously in beds ranging from the upper Tallahatta through 

 the Weches formations, the morphological and stratigraphic 

 ranges of the three species do not overlap. Similarly, transitional 

 forms linking C. smithvillensis and C. sellaeformis are unknown. 

 From independent stratigraphic evidence it is clear that the dis- 

 continuities just discussed are to be explained in terms of non- 

 deposition, erosion, and migration associated with advances and 

 retreats of the Gulf of Mexico. 



Successional Species. All the difficulties discussed above in 

 connection with transient species, with the single exception of 

 the incompleteness of the fossil record, apply with equal force to 

 successional species. In addition there is the theoretical taxo- 

 nomic problem of selecting criteria for the subdivision of contin- 

 uous lineages. From what has been said it will be clear that this 

 problem is encountered far more often in the literature than in 

 the laboratory. More extensive field work will undoubtedly bring 

 forth more examples in the near future, but it is unlikely that the 

 problem of successional species will ever be a burden to the stu- 

 dent of fossils.* 



One of the earliest and still the best example of successional 

 speciation is the fossil spatangoid sea urchin Micraster originally 

 described by Rowe ( 1899 ) . A summary of Micraster evolution 



* Successional species may, however, prove to be common in cores taken 

 from deep ocean basins where deposition and preservation may be essen- 

 tially continuous. Here lie fascinating and nearly untouched areas for taxo- 

 nomic, evolutionary, and paleoecologic research. 



