242 ACTINOZOA. 



first appear in the Chalk, while very many are 

 common to the Tertiary and Recent periods. But 

 few Recent species of Corals occur in a fossil state. 

 It appears also from the preceding tables that 

 six genera of Corals range through four periods, 

 thirty-four through three, and sixty-eight through 

 two. Some genera, however, arise in one forma- 

 tion, are apparently absent from the next, but again 

 present themselves at a subsequent period. Of 

 this seeming anomaly 'MilleYJora furnishes an ex- 

 ample. Such instances must always be received 

 with suspicion, since they are probably due to 

 defective observation. 



2, History of Zoantbaria. — 



All extinct Zoantharia belong to the group of 

 Sclerodeimata, with the exception of a few slight 

 indications of AntipatJiidm which appear in the 

 Tertiary period. The Ilalacodermata are wholly 

 recent. On the other hand, the small group of 

 Tuhulosa does not survive the Paleozoic epoch. 

 But two families of Zoantharia, Thecidce and 

 Aidoporidm, have altogether disappeared. On 

 the whole it may be said that Tahidata prevail in 

 the Paleozoic deposits, Aporosa and Perforata in 

 those which succeed. Tahidata are comparatively 

 scarce in strata anterior to the Carboniferous, 

 though no geological joeriod is without some 

 representative of this division, and in modern 

 seas four genera have been observed. A single 

 genus, Palwocyclus, which occurs in the Silurian 

 period, is the only known representative ofAporosa 

 ill strata older than the Trias. The Perforata are 

 represented in the Paleozoic rocks by two genera, 

 but, excepting these, no other forms of the group 



