60 ACTINOZOA. 



and in the autopores there are septa also, which may be 

 lamellar, spiny, or rudimentary, and are usually twelve in 

 number. Bala to Devonian. 



Halysites. Compound, corallites long and tubular, 

 arranged in a single row and united at their sides so as to 

 form laminae, which intersect ; in some species the coral- 

 lites are of two sizes. Epitheca thick. Septa absent or 

 represented by spines. Tabulae well developed, horizontal 

 or concave. Llandeilo to Wenlock. 



Distribution of the Actinozoa. 



From the point of view of their distribution at the 

 present day, the Madreporaria may be divided into two 

 groups, the solitary and the reef-building. Each of these 

 includes representatives of the three sections, Aporosa, 

 Fungida, and Perforata. The solitary corals are almost 

 entirely confined to deep water, occurring down to at 

 least 2,900 fathoms, and are found in almost all latitudes. 

 The species likewise have a wide distribution, apparently 

 not being affected by conditions of temperature and depth. 

 It might therefore be expected that they would also have 

 a long range in time ; this however is not the case, for 

 existing forms extend but a short way back into the 

 geological record, thus not a single living species is found 

 fossil in the English Cainozoic formations. The distribu- 

 tion of the reef-building corals unlike that of the solitary 

 forms is limited by both depth and temperature. Thus 

 they are found only in shallow water, not usually extending 

 lower than depths of 20 or 30 fathoms, and only where the 

 temperature of the ocean is not less than 68 F., flourishing 

 best in water even warmer than this. Like the solitary 

 corals, the reef-building genera of the present day have 



