the Limits of the Jurassic and Cretaceous Periods. 327 



Stramberg, and that their true value can only be elucidated by a 

 detailed investigation, of which I must now indicate the diffi- 

 culties. 



We already possess numerous documents upon the limestone 

 of Stramberg. From the special point of view which interests 

 us, supposing all the determinations to be accurate, and assu- 

 ming all the fossils described to be exactly contemporaneous, 

 what we know may be summed up in the following facts, which 

 appeared, a few months ago, to be the last words on the ques- 

 tion, and which, in order to preserve a chronological order, I 

 must analyze here, to discuss and modify them hereafter. 



The Cephalopoda, which have just been completely studied by 

 M. Zittel*, number fifty-five species, of which fifty are new. 

 The five others have their analogues in the Cretaceous period. 

 Not one occurs in the Jurassic stages. This proportion, if it 

 were constant, would solve the question in favour of the line A ; 

 but the other documents are far from leading to the same result. 



The Brachiopoda, w T hich have likewise been carefully investi- 

 gated, first by M. Zeuschnerf and then by M. E. Suessj, one 

 of the most competent palaeontologists in this department, give 

 a different result. Of thirty-eight species described, twenty-six 

 are new ; none pass into uncontested Cretaceous stages ; eleven 

 recur in the Jurassic period; and one (Terebratula janitor, Pict.) 

 is particularly valuable because it also occurs at the Porte-de- 

 France, and in some other deposits, of which it seems to show the 

 analogy with the limestone of Stramberg. 



The Gasteropoda are less known. M. Zeuschner§ and M. 

 Peters || have described the Nerincea of Inwald, a deposit which 

 passes as contemporaneous with that of Stramberg. They have 

 an essentially Jurassic facies. 



This is the case also with the other Gasteropoda and the 

 Acephala, at least judging from still unpublished comparisons 

 made in the magnificent collection of the Museum at Munich, 

 to such an extent that M. Zittel has concluded from these first 

 data that the Tithonian stage presents a fauna identical with 

 those of Wimmis and of Mont Saleve, which have hitherto been 

 ascribed to the Corallian stage. 



Such is the principal difficulty. If all these facts be correct, 

 we must assume that on the confines of the two periods there 

 are mixed faunas, and that the Cretaceous species, having suc- 

 cessively appeared, have gradually modified the Jurassic faunas. 



* Palaontologische Mittheilungen, vol. ii. part 1. 

 t Palaontologische Beitrdge zur Kenntniss des weissen Jurakalks. 

 t In von Hauer's Beitrdge zur Palceontologie von Oesterreich, vol. i. p. 15. 

 § " Ueber den Nerineen-Kalk von Inwald," Haidinger's Abhandlungen, 

 vol. iii. (1849). 



|| "Ueber Nerineen/' Sitzungsber. Alcad* Wiss. vol. xvi. (1855), 



