134 INVERTEBRATE PALAEONTOLOGY 



was almost restricted to that stage, disappearing in 

 the Lower Devonian. Both groups were represented 

 in the Lower Cambrian, but the early Cystids were 

 obviously the retarded survivors of a far more primitive 

 stock than the Crinoids. The simplest Cystid order 

 (Amphoridea) may well be regarded as morphogenetic- 

 ally representative of the first stage in Echinoderm 

 fixation. Most Amphoridea have ill - defined radial 

 symmetry, being irregularly sac - like forms whose 

 Echinodermal nature is apparent chiefly in stereom- 

 mesh and possession of a water- vascular-system. Arista- 

 cystis, an Ordovician genus, is the least elaborate of 

 known types ; Eocystis, a Cambrian form found usually 

 as dissociated plates, may show an early stage in the 

 development of a coherent theca. The Ordovician 

 Dendrocystis (PI. ix. fig. 6) marks a great advance on 

 the primitive condition, since it possesses a definite, 

 partly tubular, stem and an arm-like outgrowth from 

 the opposite pole. It has been found in the Caradocian 

 of Girvan; but the most familiar British member of 

 the order is Placocystis, from the Silurian. This is a 

 curiously specialized late form, laterally compressed 

 and showing diverse plating-characters on the two sides. 

 A well-marked stem is present, and two slender spines 

 seem to take the place of arms. The extraordinary 

 Cothurnocystis, recently described from the Ordovician 

 of Scotland, shares many of the features of Placocystis* 

 but is unique in the phylum in having several "oral 

 apertures." The strange specialization of these flattened 

 forms is ascribed to recumbent habits of life. 



The order Rhombifera includes the most familiar 

 types of Cystids. It shows an advance in symmetry 

 from the Amphoridean pattern, and, in later types, may 

 possess almost " crinoidal " regularity of plating. A 

 peculiar and universal feature in the order is possession 



