1865.] DUNCAN ASIATIC ECHINODBRMATA. 359 



The numerous natural-history provinces in Europe between the 

 upper member of the Neocomian and the Chalk with flints, and the 

 existence of littoral, deep-sea, and coral zones of life, prevent the 

 possibility of a close correlation of the British, Irish, French, Ger- 

 man, and Spanish Middle Cretaceous strata. This statement may be 

 carried further, and it will appear that the remoter the strata are 

 from those considered the types, the greater is the difficulty of esta- 

 bhshing a parallelism, so that at last such distant deposits as the 

 North American and South Indian present anomalies. The succes- 

 sion and parallelism of the strata do not tally in Ireland, England, 

 and Western and Southern France ; moreover there is the difficulty 

 about the relative position of the Hippurite-Chalk of Provence, Ger- 

 many, and North Africa. The restricted range of some so-called 

 characteristic species in the French Cenomanien, and their greater 

 range in the British Upper Greensand and Lower and Upper Chalk, 

 have moreover tended to complicate the parallelism of the German 

 beds containing them. It is this varying range which offers some 

 data for reflection upon the great duration of the fauna which is 

 considered characteristic of the Upper Greensand and its more or 

 less definitely parallel strata. The Hippurite-limestone of Provence 

 rests on the Cenomanien with Exogyra columha, Ammonites varians, 

 and Ammonites peramplus ; and these fossils are restricted bathyme- 

 trically to this Upper Greensand in France. In Saxony and in 

 Bohemia* these Ammonites are found in the Planer above the Hip- 

 purite-Chalk, but the Exogyra is wanting ; and thus these species, 

 which have a range in England in the Upper Greensand to well up 

 in the Chalk- with-flints, outlived a great fauna of Hippurites and 

 Corals, and witnessed great geographical changes of which the study 

 of the English series gives but little trace. The same Ammonites 

 are found above, but not below, the Hippurite-Chalk of Constantinef; 

 and the great development of this zone of Rudistes must be remem- 

 bered in estimating the time which elapsed during the changes in 

 physical geography inferred by the existence of coral and Hippurite 

 reefs preceded and followed by the conditions favourable for Ammo- 

 nite-hfe. There were, in the first instance, the conditions favourable 

 to the existence of a fauna characterized by Ammonites, Bivalves, 

 and numerous Echinodermata. In the second these conditions were 

 greatly altered, and an age of subsidence produced a deep sea with 

 the essential physical geography of a coral-sea, whose Madreporaria 

 are represented in the recent period by reef and atoll species, but 

 which were accompanied by a great fauna of Rudistes. In the third 

 instance such changes occurred as reproduced the same conditions 

 which characterized the Prehippurite sea, and Ammonites, Echino- 

 derms, and Bivalves migrated from the positions into' which they had 

 been driven by the alteration of the sea-depth. The duration of the 

 second era may be estimated by the fact that a number of species of 

 Rudistes arose, attained their greatest development, and probably 



* Coquand, Bull. Soc. Geol. de Fr. 2d ser. vol. ix. p. 340, 1853. 



t Ewald, quoted by D'Archiac, Hist. Prog. Geol. vol. v. p. 313, 1853. 



