ACTINOZOA. 61 



but a very limited geological range. As a general rule, 

 corals can only exist in salt water, there are however one 

 or two exceptions, thus Cylicia rubeola flourishes in the 

 river Thames, New Zealand. Clear water is likewise 

 generally necessary, but one species, Porites limosa, 

 thrives in muddy situations. In geological times the 

 reef-building corals had a much wider geographical range 

 than they have at the present day. Thus they occur 

 abundantly in various formations in temperate and also in 

 polar regions. But it does not necessarily follow from 

 this, that these forms required the same temperature as 

 their living representatives. 



The only extinct section of the Madreporaria is the 

 Rugosa ; this is confined to the Palaeozoic period, ranging 

 from the Ordovician to the Carboniferous. The Aporose 

 corals also appear first in the Ordovician system, the 

 earliest genus being Columnaria ; but they are very much 

 more abundant in the Mesozoic and Cainozoic periods 

 than in the Palaeozoic. The Perforata commence in the 

 Ordovician and range on to the present day, but the 

 forms occurring in' the Palaeozoic are almost entirely 

 confined to that period. The Palaeozoic families of the 

 Alcyonaria are also almost entirely unrepresented in 

 later formations, and very few of the modern forms occur 

 fossil, but Corallium is found in the Jurassic, and forms 

 allied to Pennatula and Gorgonia occur in the Cretaceous. 

 The Ctenophora do not occur fossil. The Monticuliporoids 

 (regarded by some as Polyzoa) range from the Ordovician 

 to the Permian, and they include Monticulipora, Fistulipora 

 and other forms. From all these considerations it follows 

 that there must be a marked contrast between the coral 

 faunas of the Palaeozoic and the Neozoic periods. 



Fossil corals are most abundant in calcareous rocks 



