﻿FRAAS ON THE JURA FORMATION. 



mals have by the products of their decay rendered the fine mud 

 more or less bituminous ; hence the soft shaly character of the bed. 

 With the deposition of the posidonomyce-shsle we must necessarily 

 connect that of the greyish-yellow limestones superimposed on them. 

 Although in Swabia the light-yellow calcareous marls of the Jurensis- 

 strata appear to be markedly distinct from the black posidoyiomyce- 

 shales, the boundary-line is nevertheless erased in other countries, 

 and, especially in a palseontological point of view, there exists no 

 difference between the two strata. In Calvados, in Dorsetshire, and 

 in Yorkshire there is one deposit, with the Jet Rock towards its 

 base, shaly and bituminous ; one great system of clays (in Yorkshire 

 thirty times thicker than our Jurensis-ci&ys) up to /3. of the "brown 

 Jura." Together with the saurians and fishes are already found 

 Am. radians, Am. Walcotti, Am. heterophyllus, Am. communis, Am. 

 fimbriatiis, which also continue upwards through the whole mass of 

 the clay. 



Am. Jurensis, one of the most instructive typical shells of the 

 clays of the upper Lias (£.), disappears in Switzerland and Alsace ; 

 occasional specimens are sometimes found in Mont Jura, but further 

 on in Burgundy, in the west of France, and in England it is altoge- 

 ther wanting. Am. lineatus opalinus is found instead in these 

 countries, and continues up even to the inferior Oolite. Besides this 

 there are other Ammonites belonging to the family of the lineati, 

 which in France and England represent the Am. Jurensis, as for in- 

 stance Am. comucopice and Am. Germaini. 



Am. radians is in all countries generally characteristic of the upper 

 Lias. It is especially interesting to trace the varieties of this Am- 

 monite. In the interior of Swabia some varieties of this far-spread 

 shell occur at certain places ; Am. Aalensis appears most frequently 

 in the neighbourhood of i\.alen ; there also the variety Am. radians 

 depressus, very similar to Am. j)silo?iotus, occurs ; Am. radians 

 costula belongs especially to Franconia ; and both there and in the 

 neighbourhood of Aalen Am. radians comptus occurs ; but these 

 species on the other hand are more rare at other places. In the 

 environs of Balingen (and I think that this will also be found at 

 other places by careful examination) there exist within the distance 

 of two to three leagues, distinct varieties that change with the lo- 

 calities ; how much more must this be the case in a distance of 400 

 miles ! Am. Levesquei, D'Orb., is for the most part peculiar to 

 Calvados. Am. Walcotti, which is not so prevalent in the German 

 Lias, occurs already in great number in Burgundy, and gradually 

 attains, in England, its greatest development. 



The Am. planulati of the "black Jura" are in Swabia mostly 

 crushed between the shales ; they are most perfect and elegant in Cal- 

 vados and in England. In Calvados we can collect, in the light-yellow- 

 coloured calcareous marls, thousands of Am. Hollandrei, D'Orb. 

 and Am. annulatus, Schl. Am. annulatus of Yorkshire is common 

 in all cabinets ; Am. mucronatus, Am. Requienianus, &c. of Pinperdu, 

 near Salins, are not found in Swabia, but reappear in Franconia. 



Mont Jura and the Haute-Saone possess a very peculiar Ammonite, 

 found nowhere else, Am. sternalis, D'Orb., which is certainly closely 



