i;8 INVERTEBRATE PALAEONTOLOGY 



skeletons within the sanctuary of flint-nodules constructed 

 out of the ruins of their dermal spicules (PI. ii. fig. 2). 



In parts of the Oolites, and in the Lower Cre- 

 taceous, Calcareous Sponges (especially Pharetrones) are 

 sufficiently numerous to rank as subordinate rock- 

 formers. In the Lower Bathonian of Dorsetshire Holco- 

 spongia and Raphidonema form well-marked " Sponge- 

 beds " ; while the Aptian " Sponge gravels " of Faringdon 

 are famed for the abundance of many species of the latter 

 genus. The Sycon Barroisia is fairly common in that 

 deposit. 



(D) COELENTERATA 



The Oolitic stage is the only part of the Mesozoic 

 sequence of Britain in which coral-reefs are found, 

 although scattered corals (usually solitary forms) occur 

 at all horizons from the Lias to the Chalk. Practically 

 all Mesozoic Corals are Hexacoralla ; and, in the 

 absence of Stromatoporoids and Graptolites, they con- 

 stitute most of the Coelenterate record of the era. 

 Most known families of Hexacorals appeared in the 

 Trias or Lias, and Mesozoic members of the group bear 

 a strong resemblance to those still living. The great 

 family of the Astraeidae is represented by Montlivaltia 

 among solitary types, and by Isastraea (PI. xiii. fig. i) 

 and Thecosmilia (chiefly in the Oolites) among reef- 

 builders. Of the other families, the small Anabaciidae 

 are common in the Lower Oolites, and the massive 

 Thamnasteria and Stylina are important in the Corallian. 

 The last (perhaps also the first) of the Tabulata, Chaetetes, 

 lingered throughout the era, and may be found in some 

 abundance in the Inferior Oolite of the Mendip area. 



There are but few corals in British Cretaceous rocks ; 

 they are typified by such small, solitary forms as 

 Trockocyathus and Parasmilia. But in the Upper 



