The Principles of Palaeontology. 201 



These forms are altogether characteristic of the reefs ; there are 

 of the Gasteropods, numerous Nerineas, Cerithiums, ISTaticas, 

 Turbos, Pleurotomarias ; of the Acephala, Dicer as (13 species), 

 Lima^ Pecten^ Trigonia^ Corlis; regular Echini of the family or 

 Cidaridae. Altogether more than 260 species of fossils. 



If we leave the reef of Yalfin and go eastward, we find the 

 Oolitic ooralligenous facies changing, and passing by intercala- 

 tion into marls more and more mixed with clay. We arrive at 

 deposits formed of marls and rough calcareous matter whose 

 fauna is quite different ; no more polyps are found ; Nerinea 

 and Dicer as also have entirely disappeared ; in ploce of these 

 we find fossils characteristic of the Pterocerian of the Boulonnais 

 or of the Calvados, as Pteroceras Oceania Thracia^ Pholadomya 

 and Ceromya. Some fossils common to the coral zone and the 

 muddy zone, such as Cidaris glandifera, Ostrea pulligera^ indi- 

 cate, nevertheless, as is proved by the remainder of the strati- 

 graphic arrangement, that we have here contemporary deposits. 

 The intermediate zone shows the gradual modification of the 

 fauna. When we approach the reef the Pholadomyas give place 

 to the Trigonias and Cardium; Pteroceras and Cidaris become 

 more rare, whilst the Nerineas, Diceras, and finally the Polyps 

 appear. 



This region corresponds to the lagoon region extending from 

 the barrier reefs to the shore, which was not far to the east-; 

 ward. This muddy lagoon is not favorable to the development 

 of the Polypi nor the fauna which accompanies them; the latter, 

 on the contrary, develop with vigor on the side of the open sea, 

 and their maximum of vitality is found westward of the reef^ 

 There they disappear suddenly. We are in the presence of the 

 abrupt boundary of the reef ; immediately after commences a 

 facies altogether different, which extends far toward the south 

 in the open sea ; this is the pelagic facies, characterized by Am^ 

 aa.onites {A. polyplocus, A. trachynotus\ Belemnites, Brachiopods 

 and Echini, a facies which persisted in these regions for some 

 time without modification. 



Upon examination of the succession on a vertical line at thd 

 center of the reef, we see that the advent of the muddy, waters 

 is in correlation with the sudden disappearance of the coral facies ; 

 this last, moreover, may reappear later, but in its entirety it i^ 

 carried farther toward the west of the deep-sea side. 



What has been said would apply equally, changing the name 

 of the species, to the other reefs of the Jura ; everywhere we find 

 the threefold facies ; the lagoon facies of marl, which is repre- 

 sented in other regions, as in Charente, Normandy, Barrois ; the 

 corallic facies, which has also its equivalents in Normandy, in 

 Yonne, etc.; lastly, the pelagic facies, especially developed in the 

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