THE DEVONIAN PERIOD 



135 



of them still had straight hinge lines. Many Spirifers, particu- 

 larly the wide " butterfly" genera (Fig. 74a) were common and 

 characteristic. 



b d 



Fig. 74 

 Devonian Brachiopods : a, Spirifer disjunctus; b, Spirifer inter- 

 medins; c, Stropheodonta demissa; d, Productus Hallanus. (All from 

 Md. Geol. Survey, " Devonian.") 



Mollusks. — Pelecypods and Gastropods were much like those 

 of the Silurian, though with various genera and species changes 

 (Figs. 75, 76). 



Among the chambered Cephalopods, a significant change took 

 place with the introduction of the Ammonoids (e.g. Goniatite, 

 Fig. 77) in which the septa or partition junctions were angular or 

 irregular instead of 

 simple or straight 

 as in all previous 

 forms (Nautiloids). 

 As we shall see, this 

 irregularity of par- 

 tition structure 

 gradually evolved 

 into more and more 

 complex forms, 

 reaching a maxi- 

 mum in the Meso- 

 zoic era. (See table 



O 



a b 



Fig. 75 

 Devonian Pelecypods: a, Actinopteriate textilis; b, 

 Grammysia arcuata. (From Md. Geol. Survey, 

 "Devonian.") 



