THE MISSISSIPPIAN PERIOD 157 



in accordance with the evolutionary principle already given in 

 connection with this group (see page 99). 



Arthropods. — Among Crustaceans, the Trilobites, which were 

 approaching the period of their extinction, were few in number, 

 comparatively small, and usually not highly decorated. Eucrus- 

 taceans must have been present but the records are scant. 



Among Arachnids, the remarkable group of Eurypterids had 

 notably fallen off both in numbers and size as compared with 

 the Devonian. 



Myriapods must also have been present because of their 

 existence in the Devonian and their abundance in the succeeding 

 Pennsylvanian, but fossil forms in the Mississippian are relatively 

 scant. 



Fossil Insects are not known from the Mississippian. 



Vertebrates. — Ostracoderms became extinct with the Devonian. 



Fishes. — The Selachians (Sharks), as compared with the 

 Devonian, showed an extraordinary development in numbers and 

 species. They were doubtless the most prominent of all Fishes 

 of the time, many hundreds of species being known. Teeth and 

 spines are the most numerous fossils. In sharp contrast with 

 most modern forms, many species had the mouths lined or paved 

 with rough plate-like teeth probably suitable for grinding such 

 shelled animals as Brachiopods, Pelecypods, etc. The spines 

 were doubtless provided for defence against more predaceous 

 Fishes. Dipnoans and Arthrodirans still continued though notably 

 diminished. Ganoids were still prominent, probably having been 

 more abundant than in the Devonian, with many new genera 

 and species. 



Amphibians. — As we have learned, there is good reason to 

 think that Amphibians lived in Devonian time, though actual 

 remains are not known. In the Mississippian rocks of Scotland 

 good specimens of Amphibians have been found. These all 

 belong to the long extinct and remarkable group of Stego- 

 cephalians which will be described in the next chapter because of 

 their much more satisfactory preservation in the Pennsylvanian 

 rocks. As already suggested in our discussion of Devonian Fishes, 

 it is well-nigh certain that the earliest Amphibians were derived 

 from certain types of Fishes (Dipnoans). In fact the larval forms 

 of Amphibians are true water animals breathing through gills and 

 swimming like Fishes. 



