FOSSIL ACTINOZOA. 97 



As regards the distribution in time of the above four groups, 

 the Tabulate Corals attain their maximum in the Palaeozoic 

 period, being well represented in the Silurian, Devonian, and 

 Carboniferous formations. The family Thecida is exclusively 

 Silurian ; but the familiar Seriatoporidce, Favositidce, and Mille- 

 poridiz survived to the present day; though there are many 

 breaks in our knowledge of their course. 



The Perforata are, on the whole, most abundant in Mesozoic 

 and Cainozoic strata, and attain their maximum at the present 

 day. In the Palaeozoic series the group is represented by 

 Protarea (Silurian) and Pleurodictyum (Devonian), both belong- 

 ing to the family of the Poritidce. The great family of the 

 MadreporidcR, on the other hand, did not make its appearance 

 till the Cretaceous period. 



The Aporosa are almost exclusively confined to the Meso- 

 zoic, Kainozoic, and recent periods, attaining their maximum 

 at the present day. In the Palaeozoic period the group is 

 only represented by the Silurian genus Palceocydus, belonging 

 to the Fungida. At the commencement of the Mesozoic 

 period, in the Trias, appears for the first time the great family 

 of the Astrceidce, so largely represented at the present day. 

 Almost all the Jurassic Corals belong to the Aporosa, and 

 the two families of the Turbinolidce and Oculinidtz make their 

 first appearance here. In the Cretaceous Rocks the Aporosa 

 are largely represented, the family Astraidcz being particularly 

 rich in generic forms. In the Tertiary period the group is also 

 well represented. 



The Tubtilosa, comprising the single 

 family of the Auloporidce, are exclusively 

 Palaeozoic. In the Devonian period, 

 and doubtfully in the Silurian, we have 

 the genus Aiilopora (fig. 49); and in the 

 Carboniferous Rocks occurs the genus 

 Pyrgia. There is, however, reason to 

 believe that these genera have been 

 founded upon the young and immature lg ' 49 'lvom^? 

 forms of other corals. 



ORDER II. RUGOSA. The members of this order are almost 

 entirely extinct, and, with the exception of Holocystis elegans 

 from the Lower Cretaceous rocks, and a few more modern 

 forms, are not known to occur in deposits younger than the 

 Palaeozoic epoch. With the soft parts of the Rugosa we are, 

 for the most part, entirely unacquainted, and the definition of 

 the order must therefore be founded upon the characters of 

 the corallum. The corallum in the Rugosa is highly de- 

 G 



