92 



HISTORICAL GEOLOGY 



Upper Ordovician shales that definite stages or horizons have been 

 determined largely by their use. Since the Graptolites were mostly 

 floating forms and widely distributed at a given time, they have 

 been of great value in correlating even minor subdivisions of the 

 system in such far separated regions as Great Britain, eastern 

 North America, and Australia. When it is further stated that all 

 known Graptolites are confined to the first four great fossiliferous 

 systems (Cambrian, Ordovician, Silurian, and Devonian), 1 their 

 additional importance as stratigraphic indices becomes evident. 

 In Fig. 49 the forms represent skeletons or axes of colonies, a single 



Fig. 49 

 Ordovician Graptolites: a, Tetragraptus fructicosus; b, Climacograptus bi~ 

 cornis; c, Diplograptus pristis; d, Didymograptus nitidus; e, Dictyonema 

 flabelliforme. (a, b, d, after Hall; c, after Ruedemann; e, after Matthew.) 



or a double row of protoplasmic cells having been arranged along 

 an axis. Forms with cells on both sides of the axis were very char- 

 acteristic of the Ordovician. 



Anthozoans (Corals) were common, more especially where the 

 mid-Ordovician limestones were forming. It will serve our pur- 

 pose to divide the principal Paleozoic Corals into three groups or 

 types as follows: (1) Cup Corals (solitary or compound), (Fig. 

 62a); (2) honeycomb Corals (compound), (Fig. 62b); and (3) chain 

 Corals (compound), (Fig. 62c). These Paleozoic Corals were all 

 Tetracoralla, that is, the radiating partitions (septa) of the indi- 

 viduals or polyps were four in number or multiples of four, while 

 modern Corals, which first appeared in the Mesozoic era, are 

 1 A very few Graptolites also occur in the Mississippian. 





