104 HISTORICAL PALZZONTOLOGY. 
but united together in different ways into a common mass. 
Simple corals, therefore, are the skeletons of szzgZe and inde- 
big. 43.— Zaphrentis Stokesi, a simple Fig. 44.—Upper surface of a mass of 
‘“cup-coral,” Upper Silurian, Canada. (After Stronzbodes pentagonus, Upper Silurian, 
Billings.) 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 cross-like disposition. For this 
reason, the common Lower Silurian Corals are separated to 
form a distinct group under the name of Augose 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 Aorzzontal calcareous plates or 
partitions, which divide the coral into so many tiers or storeys, 
and which are known as the “tabula” (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 ‘ tabule ” 
very highly developed. These are known as the Zadulate 
Corals ; and recent researches on some of their existing allies 
(such as /Zeliofora) have shown that they are really allied to 
