38 Environment of Tertiary Marine Faunas [236 



2. A mechanical sorting of the shells. 



This is a much more serious error and one which it is im- 

 possible to eliminate. One of the most interesting phases 

 of in-shore marine life is the dissimilarity in the dredge hauls 

 within a limited area. The in-shore currents are quite suffi- 

 cient to materially affect the character of the bottom and the 

 distribution of the algal growth and thus to limit the range 

 of a considerable number of species, particularly the vege- 

 table feeders. Unfortunately, almost all such ecologic varia- 

 tions have been washed away. Not only have near-by but 

 distinct assemblages due to slight differences in environmental 

 conditions been commingled but dead shells have been washed 

 down from the river mouths and up from the off-shores and 

 mixed together in a heterogeneous ensemble. The hard parts 

 of the smaller species, many of which constitute an important 

 item in the diet of various fishes may be carried for indefinite 

 distances beyond their normal habitat before being laid down 

 in their final resting place. In recent faunas extra-limital 

 shells are usually so badly worn that their distant origin may 

 be surmised but in the fossil forms it is much more difficult 

 to isolate them by this method. It is, however, reasonable to 

 suppose that forms occurring in any considerable abundance 

 are indigenous to the fauna but inferences made from the 

 presence of only one or two individuals should be guarded. 



B. SOURCES or ERROR IN THE DATA UPON THE EECENT 



FAUNAS. 



1. Errors in determination. 



These are much more frequent in the Recent collections 

 than in the fossil because the work has extended over a much 

 greater time and the personal element is much more conspicu- 

 ous. However, errors in the determination of the fossils are 

 frequently parallel to those of the Recent faunas so that the 

 final results are not always affected. The tendency in the 

 Recent work is towards an increasingly finer distinction of 

 species so that the ranges are becoming more and more re- 



