THE PENNSYLVANIAN PERIOD 



175 



Mollusks. — Pelecypods showed advancement in numbers and 

 species and closer approach to modern species in appearance. 



Gastropods continued 

 to be much like those of 

 earlier periods. It is im- 

 portant to note that sev- 

 eral species of the earliest 

 known land (air-breathing) 

 Gastropods (Snails) have 

 been found. 



Fig. 104 

 A Pennsylvanian Go- 

 niatite. (Goniatites 

 lyoni. After Meek.) 



Cephalopods, both 

 Nautiloids and Ammonoids 

 were much as in the Mis- 

 sissippian, though the 

 former had somewhat di- 

 minished and the latter 

 showed gradually increas- 

 ing complexity of suture 

 structure (Fig. 104). 



Arthropods. — Trilo- 

 bites were few and unimportant and close to the period of their 

 extinction. 



Eucrustaceans of Shrimp-like and Crayfish-like forms were 

 present but not common (Fig. 105). 



Arachnids were well represented by both Spiders and Scorpions, 



Fig. 103 

 Cordaites restored. (From Schuchert's 

 "Historical Geology," courtesy of John 

 Wiley and Sons.) 



