GEOLOGICAL PALAEONTOLOGY 83 



The chitinous coverings of Insects and allied groups 

 are, in respect of solubility, more durable than Molluscan 

 shells ; but their extreme brittleness and delicacy demand 

 for preservation conditions of burial that occur rarely on 

 a land-surface. Like snail-shells, elytra of beetles are 

 common in superficial soil, but such deposits only 

 occasionally persist beyond the limits of the Holocene. 

 Exceptions are found in the layers of soil covered by 

 basalt-flows in Antrim, and in the " forest-beds " of the 

 Purbeck and Pliocene. The not infrequent occurrence 

 of Arthropod remains in the Coal Measures may be 

 cited, but these nodule-cased relics had usually drifted 

 into water before preservation was accomplished. 

 Similarly the " beetle-cases " of the Great Oolite (especi- 

 ally the Stonesfield Slate) no more prove the sub-aerial 

 origin of the deposits (often replete with marine fossils) 

 than the discovery of Pterodactyle bones in the Chalk 

 indicates either terrestrial formation for that rock or 

 hydroplane habits for the Reptiles. Scorpion remains 

 in the Trias are more reliable indices of the desert- 

 conditions prevalent during that period, but even those 

 are found usually embedded in layers suggesting the 

 presence of local swamps or lagoons. 



(II) FLUVIATILE AND LACUSTRINE FAUNAS 



To a less degree than terrestrial faunas, those of fresh- 

 water are lacking in variety and durability. Mollusca 

 and Arthropods are still the dominant groups, but 

 Pelecypods support the Gastropods (still mostly 

 Pulmonates), while Ostracods and Crustaceans replace 

 the Insects. The chances for fossilization of organisms 

 in such surroundings are incomparably better than those 

 for terrestrial forms, but the porous and transitory 

 nature that belongs to most fluviatile deposits militates 



