LOWER PALAEOZOIC FAUNAS 139 



ambulacra built of single columns of plates, and 

 ambulacra composed of a few high, hexagonal plates 

 alternating, and agreeing in proportions, with the 

 contiguous interambulacrals. These qualities seem 

 certainly primitive, since they are repeated in the early 

 post-larval ontogeny of most later types ; ancient and 

 degenerate groups retain at least one single plate in 

 each interambulacrum throughout life. Two Silurian 

 genera, Echinocystis and Palaeo discus ^ from the Leint- 

 wardine Starfish bed, are somewhat problematical; in 

 their many-plated interambulacra and congested am- 

 bulacra they show progress towards normal " Perischo- 

 echinoid " characters, but their true phyletic position is 

 undetermined. Palaeechinus, a true Perischoechinoid 

 genus, has been found in the Silurian, but reached fuller 

 development in later periods. 



(F) POLYZOA 



The Polyzoa (or Bryozoa) are divided into two 

 classes: Entoprocta, in which the circlet of tentacles 

 (lophophore) surrounds both mouth and anus, and 

 Ectoprocta, with the anus outside the lophophore. The 

 latter class comprises the vast majority of known 

 Polyzoa; it is further subdivided into the groups 

 Phylactolaemata and Gymnolaemata. Save for certain 

 problematical forms, all fossil types seem referable to 

 the latter subclass. Polyzoa are usually abundant fossils 

 at all horizons from the Ordovician (in which they first 

 appear) to the Holocene ; but their normally small size 

 has occasioned relative neglect, with the result that their 

 classification is as yet indefinite. 



Of the five orders of Gymnolaemata usually recognized, 

 four are known from the Ordovician, and two are 

 virtually restricted to the Palaeozoic. The simple, 



