468 Proceedings of the Royal Physical Society. 



on a system of budding to form fixed colonies with periodic 

 liberation of sexual cells, would appear to be a more pre- 

 carious condition than in those creatures, as, for example, the 

 Hydroids, which have migratory sexual persons, or even 

 in plants, which, although fixed, have such complex con- 

 trivances to secure it through the agency of insects or 

 otherwise. 



As regards the corals, the Eugose and Tabulate divisions 

 are almost entirely confined to the Palaeozoic. They showed 

 great variety in form in the number of genera and species 

 before undergoing extinction. The Hexacoralla are first 

 found in the Trias, and have continued to the present day. 

 The genera of corals on the whole show remarkably short 

 range in time considering the low degree of organisation 

 to which they attain. The longest ranges are found in 

 Zaphrentis, Petraia, Clisiophyllum, and Strephodes amongst 

 the simple Eugose corals, with a range of Silurian to Car- 

 boniferous, and the same range is found in the compound 

 forms Cyathophyllum and Dyphyphyllum. Amongst the 

 Hexacoralla four genera only range from the Jurassic to the 

 present day. Favosites and Syringopora, of the Tabulate 

 division of the Eugosa, range from Silurian to Carboniferous. 

 The extinct Echinoderm groups of Cystoids and Blastoids 

 did not survive the Palaeozoic. The great variety of form in 

 both groups, considering the number of species that are 

 described, is very remarkable. About 50 genera of Cystoids 

 have been made for the 250 species that are known ; and 19 

 genera, with about 120 species, are recognised by Etheridge 

 and Carpenter for the Blastoids. The genera and species of 

 both Cystoids and Blastoids show very short range in time. 



The Crinoids have apparently been declining ever since 

 their maximum development in the early Palaeozoic. The 

 groups that existed during that time of dominance are 

 characteristically highly specialised and of perplexing 

 variety, and the range in time of the different forms is 

 very limited. There is one family, however, which has a 

 peculiarly long range of existence. This family is the 

 Ichthyocrinidae, which range from the Ordovician to the top 

 of the Carboniferous. The two genera Ichthyocrinus and 



