SUB-CLASS II HEXACORALLA—PORITIDAE 97 
the latter disappear; they develop a great variety of forms in the Trias, and 
from the Mesozoic down to the present day they have continued to play a 
leading part in the construction of coral-reefs. Of the six families constituting 
the Aporosa, the Astracidue is by all odds the most important and most protean, 
in comparison to which the Fungidae, Stylophoridae, Pocilloporidae, Oculinidae, 
and Turbinolidae fall into greatly subordinate rank. The other families are all 
younger than the Astraeidae, not beginning until the Jurassic, the Pocilloporidac, 
indeed, not until the Tertiary. 
The Perforata constitute a well-defined branch of the Hezacoralla, whose 
ancestry may perhaps be sought in the remarkable Archaeocyathidae of the 
Cambrian. The Hupsammidae and Poritidae occur sporadically in the Silurian 
and Carboniferous, while it is not until the Trias that the Thamnastraeidae and 
Poritidae develop a large variety of forms; from the Trias to the Tertiary, 
however, these genera continue to be important reef-builders. The Hupsam- 
nidae attain their greatest development in the Tertiary and Recent, while the 
Madreporidae belong almost exclusively to the present period. 
Occasional isolated deep-sea forms are met with in most of the several 
geological periods, but the usual mode of occurrence of the Hezxacoralla is 
associated in masses in coralline limestones ; 
the limestones may be of very variable 
thicknesses, but as a rule are interstratified 
between deposits of distinctly littoral 
character. Ancient coral-reefs most nearly 
resemble modern fringing or barrier reefs, 
but not atolls, the origin of which is clearly 
dependent upon the peculiar orographic Fic. 173 
conditions of the Pacific Ocean. Madrepora Anglica, Duncan, — Oligocene ; 
The St. Cassian, Zlambach, and Rhaetic ;} Poneitudnal Ree eae ; 
beds of the Alpine Trias contain large 
numbers of reef-building Heaacoralla ; but the pure limestones and dolomites 
of the Alps, as well as the Trias outside the Alpine region, are frequently 
either almost or entirely destitute of coralline remains. 
In the Lias coral-reefs have been found in England, Luxemburg, and 
Lorraine. Certain beds of the Dogger, usually of but meagre thickness, are 
occasionally charged with corals, as in Swabia, the Rhine valley in Baden, the 
Swiss Jura, Normandy, and England. Coralline limestones are abundantly 
developed in the Upper Jurassic of the Jura Mountains in France and Switzer- 
land, in Lorraine, Southern Baden, Swabia (Nattheim, Blaubeuern), Bavaria 
(Kelheim), many places in France and England, as well as in the whole province 
of the Alps, Carpathians, Cévennes, and Apennines; here the uppermost 
horizon (Tithonian) is especially characterised by their development. 
In the Lower Cretaceous (Neocomian) coral-reefs are found in France 
(Haute-Marne and Yonne), Crimea, and Mexico; while the Urgonian of 
Switzerland and the Bavarian Alps is occasionally charged with corals. In the 
Turonian and Senonian of the Alps (Gosau Beds), Pyrenees, and the Provence, 
numerous coral-reefs occur, usually accompanied by Ludistae ; elsewhere, how- 
ever, except in Holland (Maestricht) and Denmark (Faxoe), the Upper 
Cretaceous contains but a limited number of reef-building Hexacoralla. 
In the older Tertiary (Eocene and Oligocene) the occurrence of coral-reefs 
is restricted to the northern and southern flanks of the Alps and Pyrenees, 
VOL. I H 

