158 



Against these tropical genera, have to be set thirty-seven 

 genera of Molluscs still living in British waters, but as these 

 are existent in nearly all seas, they rather help than contradict 

 the general verdict of the local fossils. The abundance too, of 

 a rich Cephalopod fauna, as well as the number of Reptilian 

 types, tends to show the favourable conditions for marine 

 life during Liassic times. 



Another feature of the Liassic fauna is the extraordinary 

 change of life forms, in comparatively speaking, such a small 

 vertical thickness of rock. Liassic rocks a few hundred feet 

 thick show, not merely change of species, but in some 

 instances change of genera ; the differences between the 

 Redcar Rocks, and the Ironstone Series, being both striking 

 and remarkable with the genera of the Ammonites, and the 

 species of Bivalves. From the pre-glacial Crags of Norfolk 

 and Suffolk, we know that these beds indicate a marine 

 fauna similar to that of to-day, in fact the same, so far as 

 the species of Mollusca are concerned. The time, however, 

 since the Crags were deposited must be very considerable, 

 perhaps tens of thousands of years. If in the case of the 

 Crags, aftei the lapse of tens of thousands of years, little if 

 any change even in species has occurred, how long a period 

 of time must have elapsed during the deposition of the Lower 

 and Middle Lias, to permit of such great evolution of form ? 



This remarkable change in form is best illustrated by the 

 Ammonites, of which nearly 120 species have been described 

 from the Yorkshire Lias alone. As is well-known the majority 

 of the species are confined in groups to separate beds or zones. 

 Twelve such zones have been determined in the Lias of Cleve- 

 land, each characterised by a special Ammonite, or set of 

 Ammonites. Hitherto, no explanation has ever been afforded 

 of these peculiarities of geological distribution. Before any 

 solution can be attempted, the following factors ought to be 

 borne in mind. 



Each zone or bed of the Lias was deposited under 

 different conditions of sedimentation, and to these conditions 

 is undoubtedly due the ordinary palaeontological features of 

 the strata. This alone cannot have been sufficient to pro- 

 duce the great generic distinctions of the Ammonites. A more 

 important factor must have co-operated with the changing 



