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The differences of the Liassic Fauna from the present one 

 constitute those palaeontological features so characteristic of 

 the Mesozoic ages. None of the species except perhaps a 

 few Foraminifera are now living, and of these, whole groups 

 have become extinct ; the Reptilia, the Ammonites, and 

 Belemnites being the most noteworthy. With regard to 

 the Mollusca other than the Cephalopoda it has to be remem- 

 bered that, including Inferior Oolite forms, thirty-four genera 

 of them arc extinct ; whilst twenty- three genera presently to 

 be dealt with in detail although still living, are non-existent 

 in the British area. The nearest points of resemblance between 

 the living and the fossil faunas, is in the occurrence of a num- 

 ber of genera still represented on our coast, such as Mytilus, 

 Pec ten, Ostrea, Modiola, and Littovina amongst the Mollusca. 



Let us now take a brief survey of the succession of marine 

 faunas whose records are to be found in the Lias and the 

 Inferior Oolite of the district. Beginning with the Lower 

 Lias, the most remarkable zoological feature it possesses is 

 the great abundance of what are known as the Arietan Am- 

 monites, characterised by their keeled and radiately ribbed 

 shells. Arictites Bucklandi from the rocks on which Redcar 

 Pier stands, is a familiar type. Now, the Rhsetic Beds below 

 the Lias contain no Ammonites at all, yet on ascending 

 through a few feet of strata, we come across them in swarms. 

 What, therefore, is the implication of this fact ? Well, 

 probably that the conditions of life during the deposition of 

 the Trias in England, were unfavourable to their existence, 

 and that the Ammonites entered the Cleveland Liassic Sea 

 from elsewhere. The researches of an eminent continental 

 geologist, Von Mojsisovics, have shown that these chambered 

 sheila attained their maximum development, not in the Lias, 

 but in the open Triassic Seas of South Europe, Asia, America, 

 and the Arctic Regions; during which epoch over one thousand 

 species belonging to over thirty distinct genera flourished. 

 With the disappearance of the unfavourable conditions exist- 

 ing in our district during Triassic times, the Ammonites in- 

 vaded the Liassic Seas from the south, west, or east, but 

 probably not from the north, where there is supposed to have 

 been a land surface. 



Associated with the Arietan Ammonites are other genera 

 of Molluscs, including the familiar Miller's Thumb (Gryphcea 



