162 Twenhqfel and Dunbar — Modules loith Fishes. 



of the marine waters and the deposition of mud under non- 

 marine conditions. It is inferred that the beds immediately 

 succeeding Kickapoo limestone deposition record the struggle 

 for supremacy of the two rival deposits. The almost coexist- 

 ent marine organisms and thin coal beds indicate the 

 critical line between land and water and probably the tidal 

 zone. 



A peculiar fact in connection with the fossils of the nodule 

 bed is that the nodules contain almost wholly vertebrates, 

 while none have been found in the inclosing shales. This 

 could readily be explained on the assumption that the nodules 

 were pebbles and transported to the invertebrates — an assump- 

 tion, however, which is disproved by the fact that the nodules 

 show no evidence of ever having undergone transportation. 



As has been noted above, the invertebrates in the shales 

 are either small, spherical, or contain air chambers. Shells of 

 this character are readily transported for long distances by 

 currents of little velocity, the cephalopods and small shells 

 being floated and the large spherical ones rolled. Hence it 

 may be that the invertebrates were transported to the nodules. 



Many modern shores are characterized by lakes and swamps 

 in which live animals and plants that are quite different from 

 those of the adjacent shore. Many of the lakes are fresh, 

 while others are salty. Into the latter the sea effects an 

 entrance twice daily, while at some time or other many of the 

 freshwater lakes are likely to be reached by waves and tides 

 of unusual height. Into the lakes in this way are brought 

 empty shells which are irregularly distributed over the bottom, 

 thus producing the co-occurrence of animals of different 

 habitats. Elsewhere the senior author has shown that into the 

 modern peat deposits of Anticosti Island such typical marine 

 animals as echinoderms are carried by the waves.* 



It is possible that a similar state of affairs obtained at the 

 time of the deposition of the nodule bed. After the death 

 of the fishes, their decaying bodies would have precipitated the 

 matter composing the nodules. The empty shells, however, 

 would have been incapable of doing this. It would frequently 

 have been the case that a few shells would have retained 

 small amounts of organic matter and hence been able to 

 become the centers of nodules. In this way, the irregular 

 distribution of the invertebrates in the shales and their great 

 scarcity in the nodules admit of ready explanation. 



The fossiliferous bed just above the nodule layer probably 

 indicates a slight ingression of the sea. The fossils are 

 uniformly distributed and quite different from those below. 

 The animals appear to have lived where their shells are found. 



* Twenhofel, this Journal, vol. xxx, p. 67, 1910. 



