LOWER PALAEOZOIC FAUNAS 129 



cladina alone (save for one genus) appeared in the 

 Cambrian. Primitive Hexactinellids with disconnected 

 spicules (Lyssacina) are not uncommon in deep-water 

 deposits of the Lower Palaeozoic, the best known 

 British genus being Protospongia from the Menevian. 

 Long root-spicules of Hyalostelia, like those of recent 

 " Glass-sponges/' are also found. 



The Receptaculitidae, a peculiar group of organisms-of 

 uncertain affinities, are fairly abundant in the Ordovician 

 and Silurian. They are spheroidal or discoid bodies 

 showing a " honeycomb " pattern externally. By various 

 observers they have been classed as Calcareous Algae, 

 Foraminifera and Echinoderms, but they are usually 

 regarded as aberrant Porifera. Ischadites and Sphaero- 

 spongia are the chief British Lower Palaeozoic genera ; 

 the latter sometimes occurs in thick clusters in the 

 Caradocian. 



(D) COELENTERATA 



A large proportion of Coelenterata are soft-bodied, 

 and, although impressions of " Jelly Fish " and " Sea- 

 Anemones " have been found in the Cambrian of British 

 Columbia and the Upper Jurassic lithographic stone of 

 Sojenhofen, palaeontological evidence is normally re- 

 stricted to a very limited series of forms. Of the two 

 classes, Anlthozoa ajid Hydrozoa, into which the phylum 

 is divided, the^ormerVincludes many types-that-secrele 

 m assive_ mineral structures^ jDftejL-3daining very_ great 

 size (particularly in colonial forms). On the other hand, 

 most Hydrozoa possessjio skeletal .tis 

 clntinous envel_opes~o 

 evidence of the distribution and nature of the soft 



Zoantharia (C oralsj^appear first in the Ordovician, 

 except for the problematical Archaeocyathidae, which 

 occur in the Cambrian, and have been recorded from yet 

 9 



