400 PAL/EONTOLOGY 



BRACHIOPODS : Orthids and Strophomenids abun- 

 dant ; horny Inarticulata reduced in numbers ; Rhyn- 

 chonellids and Spire-bearers appear, but are rare. 



TRILOBITES of nearly all families abundant ; most 

 distinctive are the Trinucleids and Asaphids. Trilo- 

 bites may serve as zone-fossils in beds of " shelly " facies. 



3. SILURIAN. Few branched Graptolites; Mono- 

 graptus is the predominant genus, and its species 

 (chiefly) serve as zone-fossils (p. 395). 



Corals and Crinoids very abundant in the limestones. 



BRACHIOPODS : Pentamerids first appear and are 

 abundant ; Ordovician forms continue ; Rhynchonellids 

 and spire-bearers (Atrypa, Mevistina, Spirifer) common. 



TRILOBITES as in Ordovician except for absence of 

 Trinucleids and Asaphids. Encrinurids and Illsenids do 

 not survive beyond this system. 



VERTEBRATA (primitive Fishes, represented by skin- 

 teeth and fin-spines) first appear near the top of the 

 system. 



II. GENERAL FEATURES OF 



NEWER PALEOZOIC FAUNAS TAKEN 



AS A WHOLE. 



Fishes and land-plants appear and become abundant. 



Corals, Crinoids, and Brachiopods continue abun- 

 dant. Among the latter, Loop-bearers (Tevebmtnlids) 

 first appear, and spine-bearers are very abundant. 



ECHINOIDS and BLASTOIDS, previously very rare, 

 become fairly common, especially Blastoids, which, 

 however, die out in this era. 



LAMELLIBRANCHS and GASTROPODS increase in 

 numbers. 



AMMONOIDEA (Goniatites) first appear and become 

 abundant. 



NAUTILOIDEA continue common, and fully-coiled forms 

 become more frequent. 



TRILOBITES and CYSTIDS become rarer. 



Graptolites are completely extinct. 



