﻿FRAAS ON THE JURA FORMATION. 



53 



Am. heterophyllus, Am. maculatus, Am. lineatus (sometimes with a 

 ventral lobe extending upwards in the wall of the chamber) well- 

 preserved in quartz, and an abundance of Belemnites Bruguierianus 

 (paxillosus) are to be found. 



The same conditions obtain also at the other side of the Channel, 

 and the marlstone series comprehends the same that the Lias moyen 

 of the French and our/3, y. r>. contain. The Am. oxynotus and Am. 

 bifer are geologically further removed from the arietes-heds than 

 from the numismalis-ciajs, otherwise their conditions are the same 

 as those in Swabia ; only sometimes other typical shells occur, and 

 the two fossils, Terebratula numismalis and Am. Amaltheus (A. 

 Stokesi), from which the Swabian strata derive their names, are but 

 rare. In England, Gryphcea cymbium and G. gigantea are rather 

 characteristic of the middle Lias. 



If we now compare the strata in the different countries with one 

 another, the absence of Gryphcea gigantea in Swabia is remarkable. 

 In France this shell is so abundant that it is generally taken as the 

 typical shell for our a?naltheus-c\ays. D'Orbigny names it in his 

 ' Paleontologie Francaise ' G. cymbium, whilst he sets down our 

 German G. cymbium, from the Lias /3, as a variety of G. arcuata ; 

 hence the confusion experienced by the German geologist when he 

 reads of G. cymbiwn as accompanying Am. margaritatus. It must 

 be remembered, therefore, that D'Orbigny under this name compre- 

 hends G. gigantea of Burgundy, distinguished especially by the 

 concentric rings of the ventral valve, varying from an oval to the 

 most perfectly circular form, and attaining a size of 5-6 inches. 

 Whilst this shell, serving as a type in France and England, is want- 

 ing in Swabia, it happens on the other hand that Terebratula numis- 

 malis marks a sharply drawn horizon by its abundance in Swabia, 

 Switzerland, and in the east of France ; it is certainly found some- 

 times in Burgundy, but from hence towards the west of France and 

 in England it is not only no longer characteristic, but is scarcely ever 

 found in its primitive form as T. numismalis ; instead of it T. vici- 

 nalis and T. quadrifida make their appearance. The same is the 

 case with the Ammonites of the stratum ; the rich series of Swabia 

 not being found again in any other country. Am. Jamesoni, one of 

 the most frequent Ammonites in Swabia, is rarely found in France ; 

 Am. Begnardi, d'Orb., an allied form, but spinose, which is found 

 here and there in our localities, seems to take its place. On the other 

 hand, Am. armatus, Sow., is peculiar to England and seldom occurs 

 in Swabia or France ; so also Am. striatus, Zeit. (Henleyi, Beckei, 

 Sow.), which is always rare in Swabia, but is frequent in England. 

 Am. Amaltheus, the most abundant Swabian Ammonite, appears less 

 frequently further towards the west, whilst its companion Am. costa- 

 tus appears more frequently in France and England than in Swabia. 

 The most general Ammonite is Am. heterophyllus, which, however, I 

 do not regard as being the typical shell for this bed, as it is almost 

 always found where there are Ammonites. Even very lately Herr 

 v. Alberti found the genuine Am. heterophyllus in the y. of the 

 "white Jura" in the Braunenberg near Wassevalfingen. Besides, 



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