136 



HISTORICAL GEOLOGY 



a be 



Fig. 76 

 Devonian Gastropods: a, Platyceras gebhardi; 

 b, Pleuromaria capillaria; c, Loxonema hamil- 

 tonice. (From Md. Geol. Survey " Devonian.") 



on page 99.) Most of the common Nautiloid types still persisted, 

 though the simpler forms (straight and slightly curved) were 



notably diminished in 

 prominence. 



Arthropods. — 

 Trilobites showed a 

 marked decline in 

 numbers and species, 

 though they were not 

 uncommon, and were 

 still often fantastically 

 decorated. 



Eucrustaceans were 

 fairly represented by 

 several types. 



Arachnids were 

 represented by Scor- 

 pions and Eurypterids, and the Myriapods made their first known 

 appearance. Eurypterids probably culminated in the preceding 

 period, but they were still prominent and notable for great size, 

 one type having attained a length of eight 

 feet. 



Insects are not known from the Devonian. 

 Simplest Vertebrates. — Perhaps the 

 most interesting and important feature of 

 Devonian life was the profusion and de- 

 velopment of the simple Vertebrates, par- 

 ticularly the Fishes. These simple or primi- 

 tive Vertebrates are of unusual significance 

 because they were the progenitors of the 

 great groups of higher Vertebrates, which 

 gradually became more complex and diversi- 

 fied, and finally culminated in Alan him- 

 self. All known Devonian Vertebrates were 

 aquatic. 



Paleospondylus. — This remarkable creature was an exceed- 

 ingly simple and primitive type of Vertebrate. Its appearance is 

 well shown in Fig. 79. The animal, one or two inches long, 

 possessed a distinct, slender, segmented, cartilaginous vertebral 

 column supplied at one end with a rather symmetrical tail fin 



Fig. 77 

 A Devonian Goni- 



atite, Manticoceras 

 patersoni. (After 

 Hall.) 



