30 INVERTEBRATE PALAEONTOLOGY 



they are to-day. Apart from the Chalk, there are few 

 such rocks in the British sequence, although Saccammina 

 in the Carboniferous, and Numnmlina in the Eocene, 

 often give character to the beds in which they occur. 



The majority of Siliceous sponges, in spite of their 

 large size, collapse after death into a welter of slender 

 spicules, whose fate is normally like that of Radiolaria. 

 Some Lower Cretaceous cherts contain visible evidence 

 of the Poriferan origin of their silica, while the flints in 

 Chalk must have been derived largely from dissolution 

 of sponge-spicules (PI. ii. fig. 2). Calcareous sponges are 

 more massive and durable ; some bands in the Inferior 

 Oolite, and the Aptian of Faringdon, are largely built 

 of their remains. 



The reef-building capacities of Corals are well known, 

 and Madreporaria (or allied Coelenterates of similar 

 habit) have produced limestones at several stages in 

 British geological history. It must, however, be 

 remembered that a coral-reef is rarely built solely of 

 corals, while a large proportion of those animals are not 

 reef-makers. The colonial masses, and their broken 

 fragments, constitute much of a typical reef, but 

 members of other Invertebrates that sheltered- in the 

 wall, or became involved in its ruins, form a considerable 

 bulk. Calcareous algae have no small share in reef- 

 construction. The chief limestones in Britain that con- 

 tain reef-building corals are the Wenlock, Devonian and 

 Carboniferous Limestones (PL i. fig. i) in the Palaeozoic, 

 and local beds in the Lower Oolites and Corallian in 

 the Mesozoic. 



Among Echinodermata, the Crinoidea secrete the 

 most massive and extensive skeletons ; groups prevalent 

 in Palaeozoic times were especially remarkable for this 

 quality. Parts of the Wenlock and Ludlow Limestones, 

 patches of Devonian " marbles," and important masses 



