THE GRAPTOLITES AND CORALS 297 



sense than Moseley's Hydrocorallina}. Chief of these are 

 the Stromatoporoids which form an important constituent 

 of some of the Ordovician, Silurian, and Devonian lime- 

 stones. Their skeleton consists of close-set, wavy, con- 

 centric laminae traversed by abundant vertical pillars, and 

 the masses of this structure may attain a considerable 

 diameter and thickness. 



Similar structures are found here and there in later 

 systems e.g., the spheroidal bodies, about 25 mm. in 

 diameter, found in the Cambridge Greensand, called 

 Parkeria. Some fossils of this kind may, however, be 

 calcareous Algae. 



A modification of the hydroid polyp adapted to a nectic 

 life is the Medusa, or jelly-fish. These most delicate of 

 living organisms would appear to be the most hopeless 

 to look for among fossils, yet remains of such have been 

 preserved in such fine-grained sediments as the litho- 

 graphic Limestone of Solnhofen and the Middle Cambrian 

 Shale of Mount Stephen, British Columbia. 



A more advanced type of polyp than the hydroid is the 

 coral polyp, in which the internal cavity comes to be divided 

 into a central throat, leading down from the mouth, and a 

 .series of peripheral chambers separated by radiating parti- 

 tions (mesenteries). This characterizes the Actinozoa, 

 which is one of the classes of the phylum Coelenterata, the 

 Hydrozoa being another class. The common sea- 

 anemone is an example of a polyp of this advanced typa, 

 but it secretes no hard parts (stereom). Those Actinozoa 

 which do are spoken of as corals, a term which has no 



