172 Lias and 



Nautili, and bivalve shells, similar, in general grouping 

 of genera, to those of the Marlstone and Lias clay, with 

 both of which, but especially with the Marlstone, it has 

 species in common. In Yorkshire, the well-known jet 

 of Whitby is excavated from the shales on the cliffs, 

 and is formed of the fossilised stems of coniferous trees 

 that grew on the hilly islands, on the west and north. 



The remarkable assemblage of large Keptilia that in- 

 habited the Liassic seas, the number of great and small 

 Cephalopoda, including many species of Ammonites, 

 Nautili, and Belemnites, the swarms of Terebratulse and 

 Bhynchonellse, the plentiful genera and species of 

 Lamellibranchiate molluscs and of univalve shells, all 

 speak of warm seas, surrounding islands, on which 

 grew Cycads, Zamias, and other plants, that seem to 

 tell of a tropical or subtropical climate. Nor was this 

 phase of the physical geography of the time specially 

 peculiar to the Lower Lias, for it belongs alike to each 

 of the divisions, and, as we shall by-and-by see, was 

 continued into much later times. 



Nothing is more clear to me than this, that there 

 was no break in time between the successive conven- 

 tional divisions of the Lower, Middle, and Upper Lias. 

 Each in ascending succession lies quite conformably on 

 the other ; between the Lower and Middle divisions 

 there is a clear lithological passage, accompanied by 

 passage of species, and though there is generally a very 

 sudden break in lithological character between the 

 Marlstone and the Upper Lias clay or shale (due, 

 perhaps, to rapid depression of the area), yet contrary 

 to a not unprevalent belief, there is a greater number 

 of species common to these divisions than is generally 

 imagined. 



Out of 668 known species in the Lower Lias, 94, 





