BUB-CLAM ii IIKXACORALLA PORITIDAE 97 



the latter disappear; they develop a great variety of forms in the Trias, and 

 from the Mesozoic clown 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 .htr<ni<l,i<- is by all odds the most important and most protean, 

 in comparison to which the Fungidae, Stylopkoricfae, PocUloporidae, Oculinidae, 

 and Tin-l'iiii'UilfH' fall into greatly subordinate rank. The other families are all 

 vounger than the ^Idrm-iilfU', not beginning until the Jurassic, the Pocillopoi'idae, 

 indeed, not until the Tertiary. 



The Perforata constitute a well -denned branch of the Hexacoralla, whose 

 ancestry may perhaps be sought in the remarkable Archaeocyathidae of the 

 Cambrian. The I^iij>^iinmidae and Poritidae occur sporadically in the Silurian 

 and Carl M.I i if. -nms, while it is not until the Trias that the Thamnastraeidae and 

 l l nrit'nl'n> develop a large variety of forms; from the Trias to the Tertiary, 

 however, these genera continue to be important reef-builders. The Eupsam- 

 inidae attain their greatest development in the Tertiary and Recent, while the 

 Ufadrdporidae 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 HexacoraMa is 



iated 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 FIG. 173. 



Conditions of the Pacific Ocean. .V.//Tporo Anglim, Duncan. Oligocene ; 



TViP <iif faeeian 71amKanVi nnrl T?h<iAtin Brockenhurst, England, a, Calices enlarged ; 

 t. ^aSSian, ZaamDacn, ana ItnaetlC b> Longitudinal section, greatly enlarged. 



beds of the Alpine Trias contain* large 



numbers of reef -building Hexacoi'alla ; 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, CeVennes, 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 Rudistae ; 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 



