120 



COELENTERATA CNIDAHJA 



CLASS II 



only recently begun to enter extensively into the formation of coral-reefs and 



other calcareous deposits. 



In the Upper Jurassic, notably in the Tithonian of the Mediterranean 



district, certain of the Hydractinidae (Ellipsactinia, Sphaeractinia) are very 



plentifully distributed ; while, on the other hand, both the Triassic genus Heter- 



astridium, and the genera Parkeria and Porosphaera, occurring in the Cretaceous 



of Northern Europe, belong to the rarer fossils. 



Of vast importance as rock-builders, however, were the Stromatoporoidea of 



the Palaeozoic era. They occur profusely in the Ordovician and Silurian strata 

 A p of North America, England, and Russia, and 



particularly in the Middle Devonian of the Eifel 

 and Ardennes, as well as in Nassau, Devonshire, 

 . the Ural Mountains, Spain, etc. They frequently 

 attain gigantic proportions, and are an essential 

 constituent of the limestones and coral reefs of 

 the Silurian and Devonian periods ; but they 

 do not survive beyond the Palaeozoic era. 



The Gmptolitoidea, under which head the 

 Cladophom are also commonly included, are con- 

 fined to the Upper Cambrian, Ordovician, and 

 Silurian periods. They are profusely distributed 

 in the silicious schists and alum -slates of the 

 Fichtelgebirge, Thuringia, Saxony, and Bohemia. 

 They are plentiful also in the Harz, in Poland, 

 Silesia, the Baltic Provinces, and the Ural district ; 

 and again in Scandinavia, Cumberland, Wales, the 

 north of England, Scotland, and Ireland, as well 

 as in Normandy, Brittariy, Spain, Portugal, 

 Sardinia, and Carinthia. In America they are 

 found exquisitely preserved in Newfoundland, 



Canada ' New York ' Vir s inia ' Teiinessee > Ohio > 



B, Cross -section, c, Lower end, en- Wisconsin, and Iowa. They are known also in 



larged ; calcareous matter dissolved ot .LI A /T> T \ i A T i 



out by acid, v, zigzag-shaped vir*uia South America (Bolivia) and Australia, and are 



not uncommon in the drift which covers the 



ih 



Ft. 215. 



necting the virguiae ; o, Apertures plains of Northern Germany. 



(after Holm). ; 



According to Lapworth, the Graptolites are 



distributed vertically throughout six different horizons; the first of which 

 coincides with the Upper Cambrian, the three following with the Ordovician, 

 and the two uppermost with the Silurian. The Monoprionidae are especially 

 characteristic of the two Silurian horizons. 



Sub-Class 2. ACALEPHAE. Cuvier. Scyphomedusae. 

 (Discophora, Huxley.) 



Free-swimming, discoidal, or bell-shaped Medusae, with downwardly directed mouth, 

 with gastro-vascular pouches and numerous radial canals, and fairing, as a rule, the 

 margin of umbrella lobed. Cambrian to Recent. 



The Acalephs or Lobed Jelly-fishes, though frequently of considerable size, are 

 entirely without hard parts, and therefore are singularly unfitted for preservation. 



