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GEOLOGICAL MEMOIRS. 



it is subject to great changes of form, as is shown by D'Orbigny's 

 Am. Guibalianus, Am. Buvignieri, Am. Loscombi. The form with 

 wide umbilicus is abuudant at Vieuxpont ; in England it occurs 

 mostly in Lias e. and £. The I. of Swabia has produced the finest 

 specimens. The Gasteropoda, which are found in Swabia in great 

 quantities mostly silicified and small, have their greatest development 

 in Calvados. They certainly are the same species as those with us, 

 but the individuals are much larger and have been better fed, so that 

 our species of Turbo and Trochus are scarcely to be recognized again. 

 We must also acid the many new forms which have never been found 

 in Swabia, such as Euomphalus or Conus, which, as far as I know, 

 are elsewhere wanting in the Jura-rocks ; and corals, as Anthophyl- 

 lum, Lithodendron, &c, which bear evidence to conditions of the 

 sea quite peculiar and different from those found elsewhere. 



III. Upper Black Jura, e. and £. 

 Upper Lias Shale. Alum Shale. Whitby Shale. 

 Marnes du Lias supcrieur. Schistes bitumineux et Marnes a 



Trochus. Couche de ciment de Vassy. 

 Etaye toarsien (D'Orb.). 



The blackish grey bituminous schists or limestones with Posido- 

 nomyce, sharply and distinctly separated from the middle Lias, offer 

 in almost all countries a safe geological horizon. This Formation is 

 very rich in the remains of Vertebrata. The British Museum is enriched 

 with a perfect skeleton of Ichthyosaurus platyodon 25 feet long, with 

 the several well-preserved Ic. tenuirostris, with Teleosaurus Chap- 

 mani, and with numerous fishes, all from these beds at Lyme Regis 

 and Whitby. No less fortunate is Swabia, where the district of Boll, 

 Ohmden, and Holzmaden abundantly supplies the German cabinets. 

 The Saurians of Banz also and its environs, discovered and illustrated 

 by Geyer and Murk, are well-known. But besides these, doubtlessly 

 numerous other places might be found equally rich. In France one 

 of the principal localities is Vassy near Avallon (Dep. Yonne), where 

 the posidonomy cB-schists, of 12-15 feet thickness, are worked for the 

 manufacture of cement. Remains of Saurians and Fishes are here 

 frequently found, and also Am. heterophyllus, Am. annulatus, Am. 

 Deplacei, &c. Croisilles (Dep. Calvados) is not inferior to Vassy ; 

 but the development of this formation in France is not equal to 

 that in England and Germany. In Mont Jura and in Switzerland it 

 cannot always be traced ; it is often covered up or overturned for 

 miles ; other strata however share the same fate ; nevertheless at 

 Aargau, Basle, Solothurn, &c, it attains a considerable development. 

 The examination of this so important formation of the Lias in dif- 

 ferent countries seems to prove, that this deposit attained its maximum 

 development chiefly in quiet, protected bays and basins, near the 

 mouths of rivers. Saurians particularly affect such places, the em- 

 bouchures of rivers especially ; and Fishes of all kinds, Crinoidea, 

 and Cephalopoda, including Sepioe, Belemnites, and the smaller 

 Ammonites, would abound in so favourable a habitat. The dead ani- 



