UPPER PALAEOZOIC FAUNAS 163 



geny can be traced among Perischoechinoids. In the 

 one (illustrated by Lepidocentrus in the Devonian, and 

 by Perischodomus, Palaeechinus and Archaeocidaris in 

 the Carboniferous) the interambulacra became multi- 

 columnar (often with unituberculate plates), while the 

 ambulacra retained two columns with little or no 

 modification. The last-named genus seems to have 

 given rise to the Cidaroid stock (whence sprang all 

 Euechinoids) through Eocidaris and Miocidaris in the 

 Permo-Trias. The other direction of evolution (repre- 

 sented by Melonechinus (PI. xi. fig. 5) and culminating 

 in Lepidesthes) admitted some degree of complexity into 

 the interambulacra, but specialized in multiplication of 

 ambulacral columns. These large, scaly types hardly 

 survived the Carboniferous period. The anomalous 

 and energetic elaboration of Upper Palaeozoic Echinoids 

 may be regarded as an expression of phyloneanic 

 specialization. Out of a welter of unsuccessful experi- 

 ments, the efficient Cidaroid stock emerged during 

 Permian times. 



(F) POLYZOA 



The Polyzoan fauna of the Upper Palaeozoic is 

 relatively meagre. All of the orders present in lower 

 strata persisted until the Permian, but the two of chief 

 importance, Trepostomata and Cryptostomata, became 

 extinct in that period. The " Monticuliporoidea " were 

 less abundant than in the Silurian, but the Cryptosto- 

 matous Fenestellidae were dominant forms. Fenestella 

 occurs in widespread clusters in the Carboniferous 

 Limestone (particularly in North Wales and the Isle 

 of Man), and assumes almost rock-forming qualities in 

 the Magnesian Limestone of North- Eastern England. 

 Cyclostomata are generally scarce, the only group of 

 importance being the Fistuliporidae. Ctenostomata 



