﻿FRAAS ON THE JURA FORMATION. 



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latter leaning against the former. These are often excellent places 

 for the finding of fossils, whenever quartz (everywhere so common in 

 the Upper "Jura") affects the Corals and shells ; especially the old 

 pit of St. Margaret, upon the Heights between Nattheim and Hei- 

 denheim. Other such places in Swabia are the Heights near Pap- 

 pellau, Blaubeuren, Arnegg, Sirchingen near Urach, Mess-stetten, and 

 Nusplingen. Asteroid Corals [Stern-Korallen] forming these reefs 

 are Astrcsa, Mceandrina, Lithodendron, Anthophyllum, Explanaria, 

 and Agaricia ; characteristic Apiocrhiites and Cidarites are also 

 always present. But this Form of the Star-Corals is not the only 

 one ; the appearance at some places of the cellular and tubular Corals, 

 accompanied with Univalves and Bivalves, forms the transition to the 

 ^JloWwsc-facies. This Coral-group [Korallen-Form] is the more re- 

 markable, as with it is associated a Bivalve — the Biceras — which has 

 always attracted the attention of geologists. The appearance of this 

 shell is marked by two peculiarities ; it is never, for instance, found 

 without those massive cellular and tubular Corals (Columnaria and 

 Calamopora), which form entire rocks, and then only at those lo- 

 calities where some limestone overlies the " Jura." The Biceras is 

 a very rare form, and in the German "Jura" is only found at Kel- 

 heim and Batisbon, where, upon the Heights around the valley- 

 declivities of the Danube, Laaber, and Naab, a series of snow-white 

 limestones, full of remarkable forms which are found nowhere else in 

 the " Jura," lie above the Dolomite. Complete specimens, however, 

 are very rare, and of these most are merely casts. Chama Miinsteri, 

 Biceras arietina, Terebratida inconstans, Lima gigantea, Mytilus 

 ampins, Nerincea, Natica, Pteroceras, &c. are most generally found. 

 The rarer forms are Caprotina-\ike Bivalves and Modiola lithophaga ; 

 amongst the Gasteropoda, Tornatellce and Nerincece, reminding us of 

 Gosau and Abtenau ; and lastly, Pollicipes Nilssoni and P. rigidus 

 at Ebenwies. These .ZHcems-limestones are not separated from the 

 underlying Dolomites ; there is an almost imperceptible transition of 

 the Dolomite into the Limestone, and frequently the Biceras is seized 

 upon by the former. No other Jurassic bed overlies the Limestone, 

 but above it we find the Greensand-strata with the well-known Exo- 

 gyra columba, and we clearly perceive that the Bicer tfs-limestones form 

 the only and last Jurassic strata between the Dolomite and the Chalk. 



On comparing the phsenomena of the French "Jura" with this 

 locality, we are struck with the similarity of the formation. Between 

 the celebrated grotto of Arcy (Yonne) and Chatel Censoir, we ascend 

 the mountain over massive rocks containing Am. planulati and Tere- 

 bratula insignis ; arrived at the summit, we find it covered with snow- 

 white, softish limestone, consisting almost entirely of cellular and 

 tubular Corals, Nerincece, and Bicer at a. In the vicinity, however, 

 the Chalk-formation, which surrounds the whole Paris-basin, crosses 

 the Yonne, and further on the Loire. The conditions of the Mor- 

 tagne de la Sarthe, from whence are obtained the pretty little Bicer as- 

 casts, are the same. The Biceras is also found at Pagnoz in Mont 

 Jura and in the Swiss " Jura," in the vicinity of the Neocomian. 

 But in the Swabian " Jura" we may in vain look for its existence, 

 because here there is not a trace of chalk, and the Star-coral^Bc?e« 



