MATERIALS 31 



of the Carboniferous Limestone (PL i. fig. 2), are 

 replete with Crinoid columnals almost to the exclusion 

 of other material. In Mesozoic rocks remains of Crinoids 

 are often abundant, but they are rarely so profuse as 

 to make up much bulk of the strata in which they occur. 

 Thin sheets of sparry limestone in the Lower Lias, 

 representing individuals or clusters of Pentacrinus, and 

 beds of Millericrinus in the Bathonian (PI. i. fig. 3), afford 

 exceptions to this general rule. Echlnoids, though often 

 common fossils, never attain the rank of rock-formers 

 in this country. 



The flimsy colonies of calcareous Polyzoa are often 

 abundant, but it is rare to find them in the role of 

 rock-formers. Fenestella sometimes crowds layers of 

 Carboniferous and Permian Limestones ; while more 

 massive " Corallines " are important constituents of the 

 Faringdon Greensand and Pliocene Crags. 



Brachiopoda flourished to such a degree in the 

 shallower Palaeozoic seas that they rank as the chief 

 "shells" of that epoch. In consequence, the equivalents 

 of modern shell-beaches were made of their remains. 

 Various members of the Orthacea convert some layers of 

 Ordovician sandstone into local limestones, Pentamerus 

 does the same in the Llandovery, Leptaena and Atrypa 

 similarly constitute parts of the Wenlock, while Con- 

 chidium is largely responsible for the existence of the 

 Ludlow Limestone (PI. ii. fig. i). Many masses of 

 Devonian Limestone are replete with Brachiopods of 

 various genera, while the broken shells and spines of 

 Productus give character locally to the Carboniferous 

 Limestone. In later periods Brachiopods were, on the 

 whole, less numerous, though certain genera of Rhyn- 

 chonellacea and Terebratulacea give a distinctive ap- 

 pearance to some Jurassic Limestones, and constitute no 

 small proportion of their bulk. 



