THE LOWER SILURIAN PERIOD. 105 



ing at its summit a more or less deep depression (the "calice") 

 in which the digestive organs are contained. The space within 

 the corals is divided into compartments by numerous vertical 

 calcareous plates (the "septa"), which spring from the inside 

 of the wall of the cup, and of which some generally reach the 



Fig. 43. Zaphrentla Stokesi, a simple Fig. 44. Upper surface of a mass of 

 " cup-coral," Upper Silurian, Canada. Slrombodes pentagonua, Upper Silurian, 

 (After Billings.) Canada. (After Billings. ) 



center. Compound corals, again (fig. 44), consist of a greater 

 or less number of structures similar in structure to the above, 

 but united together in different ways into a common mass. 

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

 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 cross-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 



