ORDOVICIAN FOSSILS 



45 



The Commoner Fossils 



PLANTS. No undoubted seaweeds have been found in our Ordovician rocks. "Fucoids," 

 which were once thought to be seaweeds, turn out to be a variety of other things. 



PROTOZOA. No protozoans have been found in our Ordovician rocks. Foraminifera have 

 been found sparingly in Ordovician rocks elsewhere and may yet turn up in Ohio. 



PORIFERA . Sponges are not abundant in our Ordovician rocks and, in fact, the common- 

 est of these, Stromatocerium huronense , may not be a sponge at all. It grows in irregular 

 colonies covered with pimple -like bumps. It is sometimes mistaken for a coral but can easily 

 be distinguished from it because it shows no tubes in cross -section. This doubtful sponge is 

 found in the Richmond. 



ANTHOZOA. Both horn corals and colonial corals are found in our Ordovician rocks. 

 The commoner kinds may be recognized by means of the following key. 



Key to the Commoner Ordovician Corals of Ohio 

 ( see p. 4 for use of keys ) 



1. a) Animals in colonies of closely packed tubes 2 



b) Animals solitary: horn corals Streptelasma 



2. a) Colonies hemispherical 3 



b) Colonies in thin, irregular crusts Protarea 



3. a) Tubes large, numerous 4 



b) Tubes small (diameter of an ordinary pin or less) Tetradium 



4. a) Tube walls without pores Columnaria 



b) Tube walls with pores Calapoecia 



The commonest of the horn corals belong to the genus Streptelasma (fig. 49) and may 

 be recognized by their horn-like shape. Some people still call these fossil cow horns. Broken 

 and worn specimens are not obviously horn-shaped; they may be recog- 

 nized by cross -sections of the septa or partitions which radiate outward 

 from the center of the cup. The commonest species is Streptelasma 

 rusticum which occurs, often abundantly, throughout the Richmond. 



The colonial corals are found either as hemispherical colonies or as 

 thin, irregular crusts growing on any convenient surface. 



The hemispherical colonies with numerous large (l/8 to 3/16 inch) 

 tubes with strong, short septa, are Columnaria alveolata (fig. 50) of the 

 Richmond, especially the upper formations. 



Calapoecia cribriformis is like Columnaria but with rather large 

 pores in the walls of the tubes, arranged in rows between two septa. 

 This feature is not visible in entire colonies but can be seen when a 

 colony is broken parallel to the tubes; it also occurs in the Richmond, 

 especially the upper formations. Tetradium approximatum is like Col- 

 umnaria but has very narrow tubes; it occurs in large hemispherical 



Fig. 49 



