LOWER PALAEOZOIC FAUNAS 135 



of stereom- folds matching on contiguous plates, and 

 becoming localized into " pectinirhombs " in specialized 

 forms. Macrocystella^ a crinoid-like form with a long 

 stem and five bifurcating arms, is not uncommon 

 (as dissociated plates) in the Shineton Shales. The 

 Ordovician Caryocrinidae show much superficial resem- 

 blance to Crinoids and Blastoids. Echinosphaera, an 

 almost globular form with numerous " rhombiferous " 

 plates, is fairly abundant in the Ordovician, but is 

 rarely found entire in this country. Silurian Rhombi- 

 fera show obscure parallelism with contemporaneous 

 Amphoridea in shape. Schizocystis and Lepadocrinus^ 

 with recumbent "arms " suggestive of Blastoid ambulacra, 

 are among the few Cystid genera found in good preserva- 

 tion in Britain. They occur in the Wenlock Limestone. 



A small and inconspicuous order, the Aporita, occurs 

 in the Ordovician and Silurian. Cryptocrinus, its typical 

 representative, has a theca comparable with a crinoid 

 calyx, but phyletic connexion between the two groups is 

 very improbable. 



The Diploporita, in which paired pores perforate the 

 median stereom -layer, are perhaps the most interesting 

 order of Cystids, since Blastoids were certainly derived 

 through them, and there is reason to consider them 

 ancestral to Crinoids as well. The order is essentially 

 Ordovician in date, and seems clearly descended from 

 the Amphoridea. The theca is usually globular or pen- 

 tamerous in symmetry, with irregular plating (becoming 

 more definitely arranged in later types) and food-grooves, 

 terminating in brachioles, passing with increasing pen- 

 tamerous regularity over the surfaces of the plates. 

 Sphaeronis has food-grooves extending scarcely beyond 

 the mouth; Glyptosphaera has five branching grooves 

 passing for some distance over the theca. Mesocystis 

 and Proteroblastus show a strong tendency to stereo- 



