104 



HISTORICAL PALEONTOLOGY. 



but united together in different ways into a common mass. 

 Simple corals, therefore, are the skeletons of single and inde- 



Fig. 43. Zaphrentis Siokesi. a simple 

 "cup-coral," Upper Silurian, Canada. (After 

 Billings.) 



Fig. 44. Upper surface of a mass of 

 Strombodes pentagomts, Upper Silurian, 

 Canada. (After Billings.) 



pendent polypes ; whilst compound corals are the skeletons of 

 assemblages or colonies of similar polypes, living united with 

 one another as an organic community. 



In the general details of their structure, the Lower Silurian 

 Corals do not differ from the ordinary Corals of the present 

 day. The latter, however, have the vertical calcareous plates 

 of the coral (" septa ") arranged in multiples of six or five ; 

 whereas the former have these structures arranged in multiples 

 of four, and often showing a cress-like disposition. For this 

 reason, the common Lower Silurian Corals are separated to 

 form a distinct group under the name of Rugose Corals or 

 Rugosa. They are further distinguished by the fact that the 

 cavity of the coral (" visceral chamber ") is usually subdivided 

 by more or less numerous horizontal calcareous plates or 

 partitions, which divide the coral into so many tiers or storeys, 

 and which are known as the "tabulae" (fig. 45). 



In addition to the Rugose Corals, the Lower Silurian rocks 

 contain a number of curious compound corals, the tubes 

 of which have either no septa at all or merely rudimentary 

 ones, but which have the transverse partitions or " tabulae " 

 very highly developed. These are known as the Tabulate 

 Corals ; and recent researches on some of their existing allies 

 (such as Helicpora) have shown that they are really allied to 



