138 INVERTEBRATE PALAEONTOLOGY 



Dicyclic Crinoids are divided into Inadunata, Flexibilia 

 and Camerata. The first and last orders are parallel, 

 and practically contemporaneous, with the similarly 

 named divisions of Monocyclica ; an " Adunate " group 

 might well be established for the Dicyclic Crotalo- 

 crinidae. Apart from the Silurian Crotalocrinus (one of 

 the commonest of Wenlock Crinoids), Cyathocrinus (PL 

 ix. fig. 7), GissocrinuS) Petalocrinus and Dendrocrinus are 

 the most familiar Inadunate Dicyclica of the Lower 

 Palaeozoic. Reteocrinus was an Ordovician precursor 

 of the small, mostly Carboniferous, Camerate group. 

 Flexibilia are chiefly Mesozoic and Recent forms, but 

 Taxocrinus is not uncommon in the Wenlock Limestone. 



Lower Palaeozoic Eleutherozoa are rarely abundant, 

 and usually obscure. Wonderfully perfect impressions 

 (almost amounting to petrifactions) of apparently pelagic 

 Holothurians, discovered in the Middle Cambrian of 

 British Columbia, carry back the history of that debat- 

 able class to very early times. The two main sections of 

 Stelleroidea (Asteroids and Ophiuroids) are both repre- 

 sented in the Lower Palaeozoic (the former in the 

 Cambrian), but their collapsible nature makes inter- 

 pretation of their structures and affinities difficult. The 

 chief British Proterozoic deposits in which Stelleroids 

 are at all numerous are thin layers in the Caradocian 

 of Girvan and the Lower Ludlow of Leintwardine 

 (Herefordshire). Lapworthura, an " Ophiuroid " form, 

 is extraordinarily abundant in the latter " Starfish bed." 

 Isolated occurrences in Ordovician and Silurian rocks 

 of other British localities are not infrequent, but the 

 usual obscurity of the fossils makes them unsuited for 

 discussion in general terms. 



The earliest recorded Echinoid is Bothriocidaris, a 

 small, subglobular form from the Ordovician. It differs 

 from all other members of the class in having inter- 



