120 COELENTERATA—CNIDARIA 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, Sphueractinia) 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 Strom«atoporoidea of 
the Palaeozoic era. They occur profusely in the Ordovician and Silurian strata 
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 Graptolitoidea, under which head the 
Cladophora 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, Brittany, Spain, Portugal, 
ee re Sardinia, and Carinthia. In America they are 
ee etnaon gaizlanes, Barr. Silurian. found exquisitely preserved in Newfoundland, 
A, Specimen from silicious schists of 
Feuguerolles, Calvados; natural size. (% Jew ek. Vireini ; i 
Bie olmeie tion Mou Swelce: Canada, New York, Virginia, Tennessee, Ohio, 
Bs Cross-section, C, Lower end, en- Wisconsin, and Iowa. They are known also in 
arged; calcareous matter dissolved - F wt ic A 
out by acid. 2, Ziezse-shaped virgula: SOUth America (Bolivia) and Australia, and are 
v’, Rod-like virgula; th, Conjoined . a thi rer 
walls of hydrothecae; ¥, Crossbarscon- 0b uncommon in the drift which covers the 
necting the virgulae; 0, Apertures plains of Northern Germany. 
(after Holm). 3 ‘ : 
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 J/onoprionidue are especially 
characteristic of the two Silurian horizons. 

SOON 
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SSN 
Bre eRe e Setians 




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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 having, 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. 
