CHAPTER IV 



MESOZOIC FAUNAS (PLS. xm. AND xiv.) 

 (A) GENERAL ACCOUNT 



WITH the exception of deposits formed under 

 abysmal conditions, almost all types of sedimen- 

 tary rocks occur in the British Mesozoic sequence. 

 Desert sands, marls accumulated in inland (often acrid) 

 water, fluviatile, deltaic and marine strata constitute the 

 rocks at various horizons and localities. Further, the 

 deposits of marine origin range from littoral sands and 

 oolites through argillaceous series to true pelagic ooze. 

 Opportunities are thus afforded for the preservation of 

 very varied faunal assemblages, while the recurrent local 

 variations of physiography provide more or less con- 

 tinuous records of several types. 



Of the four periods into which the Mesozoic era is 

 divided, the two earliest are extremely unsatisfactory 

 (from the standpoint of Palaeontology) in their British 

 development. Practically all parts of the Triassic 

 system are virtually unfossiliferous ; while the Rhaetic 

 is represented by a layer rarely exceeding twenty feet in 

 thickness. By way of compensation, the Jurassic and 

 Cretaceous strata are not only highly fossiliferous, but 

 have retained their organic contents in an admirable 

 state of preservation. Fossils from the Lias, Oolites and 

 Chalk attract attention by their intrinsic beauty, and 

 often form no inconsiderable bulk of the heterogeneous 

 collections of curiosities displayed in the cabinets of 

 Victorian drawing-rooms. 



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