THE MISSISSIPPIAN PERIOD 



375 



tionships between these primitive amphibians and the fishes 

 are so close as to leave small doubt that they were actually 

 derived from one of the groups of fishes in the Devonian. 

 Indeed, even among the highest amphibians the young are 

 hardly more than fishes, breathing water through gills and 

 swimming by means of fins. 



FIG. 394. Larval form of a salamander, showing the finlike fringe on the 

 tail and the branching external gills just behind the head. 



QUESTIONS 



1. Many limestones such as those of the Mississippian system 

 contain nodules of flint and chert, a very dense form of quartz. 

 During the weathering of the limestone what should become of 

 these nodules ? 



2. What different types of animals could make five-toed foot- 

 prints ? Which of these groups is the lowest in the scale of evolution ? 



3. An important part of the salt now used in the United States 

 is obtained from Mississippian strata. From your knowledge of 

 the formations of this age, where should you expect to find the 

 center of this salt industry ? 



4. What structure should you expect to find in the Paleozoic 

 rocks of Belgium ? Why ? 



5. At a locality in Illinois the 

 Pennsylvanian and earlier Paleo- 

 zoic systems have the relation 

 shown in Figure 395. What 

 events are indicated ? 



6. On what grounds is it jus- 



FIG. 395. Section of Paleozoic beds 

 in Illinois. 



tifiable to separate the Mississippian as a distinct period ? 



