32 INVERTEBRATE PALAEONTOLOGY 



After the Palaeozoic epoch, Mollusca assumed the 

 mantle of the Brachiopoda. In particular, the Ostreidae, 

 which even to-day occur in congested clusters round 

 our shores, are responsible for much of the Middle Lias 

 of Yorkshire and the Forest Marble of the Southern 

 Counties. Ammonites are rarely rock-formers, but at 

 some horizons in the Inferior Oolite of Dorsetshire (e.g. 

 at Burton Bradstock and Bradford Abbas) they are so 

 abundant that the scanty matrix scarcely separates the 

 shells. In Tertiary times, Molluscan life in the British 

 area was rarely of such exuberance as to mask the 

 character of the sediments, but the well-known Oyster- 

 bed at the base of the Reading series, the " basement- 

 bed " of the London Clay, layers in the Bracklesham and 

 Barton series, the Oligocene (PI. ii. fig. 3), and parts of 

 the Crags, owe much of their bulk to the abundant 

 Pelecypoda and Gastropoda. 



Trilobites are often disseminated (in a more or less 

 broken state) in great profusion in Lower Palaeozoic 

 rocks, and even in the Carboniferous Limestone, but 

 they can hardly be classed as rock-formers. The most 

 efficient Arthropoda in this respect are the Ostracoda. 

 Beyrichia in the Ordovician and Silurian, and Cypridea 

 in the Wealden, often occur in swarms to the virtual 

 exclusion of matrix ; but their small size and impersistent 

 deposition restrict their rock-forming capacities to thin 

 beds. Examination of prepared " flint-meal " shows 

 abundant Ostracoda in the Chalk ooze ; their remains 

 are usually subordinate to Foraminifera and Sponge- 

 spicules. 



(Ill) FOSSIL-BANDS (PI. vi.) 



Brief experience of fossil-hunting suffices to show that 

 organic remains are rarely scattered evenly through a 

 deposit, but tend to occur in more or less defined bands. 



