160 INVERTEBRATE PALAEONTOLOGY 



known as Pleurodictyum (occurring as casts including an 

 apparently symbiotic worm) is a well-known Middle 

 Devonian fossil ; the genus lingered into the Lower 

 Carboniferous, in which stage the massive, honeycomb- 

 like colonies of Michelinia were the most effective 

 representatives of the family. Syringopora, a form 

 similar to the recent Tubipora in habit, is locally 

 abundant in the Lower Carboniferous ; while Chaetetes, 

 the only Tabulate to survive beyond the Palaeozoic, 

 sometimes builds massive sheets of limestone. 



Among Hydrozoa, the Stromatoporoidea were of first 

 importance in Devonian reef-limestones, but suffered 

 extinction in the Carboniferous. Dendroid Graptolites, 

 which may either be persistent types or reversionary 

 mimics of early forms, are to be found, though rarely, 

 in the Devonian and Carboniferous ; while the essentially 

 Silurian genus Monograptus has been recorded from the 

 Devonian. Sporadic discoveries of such a nature fail to 

 detract from the value of the Graptolites as indices of 

 Lower Palaeozoic horizons. 



(E) ECHINODERMATA 



In respect of Echinodermata, the Upper Palaeozoic 

 could be described as the " age of Blastoids, Camerate 

 Crinoids and Perischoechinoids." These types all had 

 representatives in the Silurian, but none survived the 

 Permian. The extreme scarcity of Echinoderms in the 

 last Palaeozoic period is somewhat mysterious, but it 

 helps to emphasize the contrast between Carboniferous 

 and Mesozoic types. 



Late forms of Rhombiferan Cystids, and perhaps of 

 Amphoridea, have been recorded from the Devonian, 

 but they are inconspicuous comparable with surviving 

 Graptolites in stratigraphical insignificance. Minute, scaly 



