THE MAIN KINDS OF FOSSILS 



35 



probably an extinct offshoot of the Hydrozoa. The shell is like a sharp cone or pyramid with 

 regular transverse ribbing. They are generally scarce from the early Cambrian to the Triassic 

 but are common enough in some formations to attract attention. 



CLASS STROMATOPOROIDEA . This is an extinct group of coral-like animals in which 

 the skeleton is made up of layers of calcium carbonate perforated by minute holes. The stroma - 

 toporoids were exclusively marine and lived from Ordovician to Cretaceous times. 



Stromatoporoids are common fossils in the Silurian areas of western Ohio. Some colonies 

 have become silicified, entirely or in part, and they weather out of the rocks as rounded masses 

 the size of a baseball up to that of a football. The interior of these colonies maybe stained bright 

 red and brown. They are also abundant in the Devonian limestones and contributed much mater- 

 ial to the Devonian reefs. 



CLASS SCYPHOZOA 



These are the jellyfish, umbrella-like animals with a fringe of 

 Most of the jellyfish are marine but there are a few freshwater 



tentacles but no hard parts, 

 representatives; one of the latter is sometimes common in Ohio ponds at the present time. The 

 jellyfish first appear in the Lower Cambrian and they are preserved under exceptional circum- 

 stances in other parts of the geologic column. 



CLASS ANTHOZOA . This class includes the 

 corals and the sea anemones. Most of the living 

 corals are colonial but a large number of the fossil 

 ones were solitary (fig. 33). The colonial forms need 

 no description; the solitary forms secreted a cup-like 

 shell which is divided internally by radiating partitions, 

 called septa, characteristic of the class. The shell of 

 the coral is solid enough to be preserved; fossil corals 

 are abundant in many sedimentary rocks and some of 

 them are good index fossils. The Anthozoa are exclu- 

 sively marine and are found from the Ordovician to 

 the present. 



In Ohio, corals are most abundant in the Ordo- 

 vician, Silurian, and Devonian rocks. They are rarer 

 from the Mississippian to the Permian and are not 

 found, of course, in the Pleistocene deposits, which 

 are entirely non-marine. 





Fig. 33 



Phylum Platyhelminthes 



All worms are not like the earthworm that everybody knows. The animals we call "worms" 

 are so diverse that they make up several phyla of which the Platyhelminthes, or flatworms, are 

 only one. The flatworms are common at present, both in the sea and in fresh water, or as para- 

 sites of other animals, including man. They have no hard parts that could be preserved and 

 therefore their fossil record is very scanty. No fossil flatworms have yet been found in Ohio. 



Phylum Nemathelminthes 



The threadworms are distinct enough from the flatworms and other worms to be set apart 

 in a phylum of their own. Like the flatworms they have no hard parts and their fossil record is 

 negligible. There are no records of fossil threadworms for Ohio. 



