460 



HISTORICAL GEOLOGY 



sutures. The straight (Orthoceras) and coiled (Gomphoceras) ceph- 

 alopods with simple sutures continued throughout the period but 

 were much less common in the later portion. 



Arthropoda 



Trilobites. — During the earlier stages of the Devonian more than 

 50 species of trilobites (Fig. 435 A—C) are known to have existed, 



Fig. 435. — Devonian trilobites: A, Lichas {Gaspelichas) jorillonia (cephalon) ; 

 B, Dipleura dekayi; C, Phacops rana. 



but the numbers rapidly decreased during the later stages. In the 

 earlier portions of the period especially, a number of highly orna- 

 mented, spinous forms (Fig. 435 A) lived, but later these extrava- 

 gant species largely disappeared, and those of simpler outlines 

 remained. The decline of the trilobites during 

 the Devonian was very marked, and at its close 

 they were on the verge of extinction, although 

 a few survived until the close of the Paleozoic. 

 Other crustaceans (Fig. 436) also lived in con- 

 siderable abundance. 



Barnacles, which are retrograde crustaceans 

 that have given up a free-moving existence for 

 a stationary one in which they are protected 

 by a calcareous covering, began in the Ordo- 

 vician, but the common acorn barnacle began in 

 this period. 



Eurypterids attained their greatest size during 

 I^._ the Devonian, one species reaching a length of 

 almost eight feet (Fig. 437), 



n6- — Echint 

 caris punctata, a 

 vonian crustacean 



