UPPER PALAEOZOIC FAUNAS 161 



Edrioasteroids, usually cemented by the aboral surface, 

 perpetuated the class (or order) beyond the Devonian. 

 Agelacrinus is typically Devonian, and Lepidodiscus has 

 been found in the British Lower Carboniferous. 



Blastoids were pre-eminently Devono-Carboniferous 

 forms. Like other recognizable fossils, they are rare in 

 the British Devonian, but Orbitremites (PL xi. fig. 3) 

 and Phaenoschisma may be found in local " nests " in 

 parts of the Carboniferous Limestone. The Bolland 

 district of Lancashire is most prolific in these small 

 forms. The Devonian Nucleocrinus is a precursor of 

 the former genus; Pentremites, from the American 

 Sub-Carboniferous (showing " petaloid " ambulacra and 

 elaborate hydrospires), is more nearly related to the latter. 

 The last known Blastoid died in early Permian times. 



Inadunate Monocyclic Crinoids persisted in their 

 unaspiring course. Symbathocrinus is not uncommon 

 in the Carboniferous Limestone. Both Adunata and 

 Camerata failed to survive the Carboniferous period, 

 but in their short careers attained great abundance. 

 Hexacrinus is a well-known Devonian Adunate ; Platy- 

 crinus (PL xi. fig. 4) is one of the commonest Crinoids 

 of the Carboniferous Limestone. Acrocrinus, one of the 

 latest Adunata, is remarkable for inclusion of numerous 

 accessory plates between the basal and radial circlets, 

 giving a spurious " Camerate " appearance to the calyx. 

 True Monocyclic Camerata were the chief rock-formers 

 among Echinoderms. They attained their acme, and 

 reached extinction, in the Carboniferous. Melocrinus is 

 a familiar Devonian type on the Continent, but from 

 the British standpoint Actinocrinus and Amphoracrinus 

 (exclusively Carboniferous genera) are the most im- 

 portant. Columnals of these forms constitute much of the 

 Carboniferous Limestone (PL i. fig. 2), and their massive, 

 tegminate crowns are locallyabundant and well-preserved, 

 ii 



