MESOZOIC FAUNAS 175 



limestones in the Oolitic series. Throughout the Jurassic 

 period conditions comparable with those of the Coal 

 Measures (on a small scale) obtained around the 

 Highland coasts ; and estuarine deposits form the bulk 

 of Lower Oolitic strata in Northern England. Towards 

 the close of the Oolitic stage delta-formation was shifted 

 to the southern area, and the huge, but local, mass of 

 the Purbeck-Wealden series was formed. 



The Cretaceous period was marked by the commence- 

 ment of gradual and continuous depression, whereby 

 the amount of land in the area was steadily diminished. 

 Although its main coastline seems not to have passed 

 much farther westwards than that of the Rhaetic, the 

 Cretaceous sea became open by submergence of the 

 " Eastern Isle." Littoral facies of Upper Cretaceous 

 rocks remain in such fragmentary deposits as those of 

 Haldon Hill in Devonshire, the Hibernian Greensand of 

 Antrim, and outliers in and near the Isle of Mull. But 

 calcareous ooze, often almost free from detrital matter, 

 represents the typical product of this phase. Upheaval 

 brought these thalassic conditions to an end, but the 

 stages of its operation are lost (in Britain) by much 

 denudation of the Upper Chalk in pre-Eocene times. 



The Mesozoic era is often called the " age of Reptiles," 

 and no more appropriate designation could be applied. 

 But there are many features in the Invertebrate fauna 

 of the era that are equally as distinctive, if not so 

 superficially evident, as the reptilian regime. In Britain 

 there is a particularly marked contrast between the 

 faunas of the Upper Palaeozoic and Mesozoic strata. 

 This diversity is not entirely due to the long Permo- 

 Triassic interval during which very scanty palaeonto- 

 logical records are available. In other regions, where 

 fossiliferous Permian is overlain by equally prolific 

 Trias, the extinction of old types and inception of 



