1 82 INVERTEBRATE PALAEONTOLOGY 



radicles. Both of these qualities were carried far during 

 the Mesozoic, but have been yet more elaborated in 

 later times. Small, somewhat obscure, forms, usually 

 ascribed to Acrosalenia and Hemipedina, occur in the 

 Rhaetic and Lias. The Inferior Oolite is rich in species 

 of Pseudodiadema^ Hemipcdina and Stomechinus ; Hemi- 

 cidaris and Acrosalenia are important in the Bathonian. 

 The Corallian is one of the most prolific horizons for 

 Diademoida (coral-reefs are favourite resorts of modern 

 types) ; Hemicidaris and Diplopodia are represented by 

 abundant and characteristic species. The latter genus, 

 with small forms of Salenia and Peltastes, predominated 

 in the Lower Cretaceous ; Phymosoma (PI. xiii. fig. 4), 

 with greater ambulacral complexity than is usual in 

 Mesozoic forms, is sometimes common in the Chalk. 

 The probably regressive Echinothuriidae, represented 

 by the " collapsible " genera Asthenosoma and Phormo- 

 soma at the present day, were foreshadowed by Pelan- 

 echinus in the Corallian and Echinothuria in the Chalk. 



The " Regular " orders were joined by their " Irregular" 

 offspring in the Lias, if not before. Two distinct orders, 

 with very different morphogenetic qualities, are certainly 

 known from that period. Holectypoida, the least 

 " Irregular " of their kind, are represented by Plesiechinus, 

 which became abundant in the Bajocian ; Pygaster con- 

 tinued this stock in the Corallian. Holectypus, differing 

 from the Pygasteridae in the posterior position of the 

 periproct, is known from the Upper Lias, and has 

 abundant species in the Lower Oolites, Cornbrash and 

 Corallian. Discoidea, an important arid ambulacrally 

 elaborate descendant of Holectypus, is essentially a 

 Lower Cretaceous and Lower Chalk genus ; Conulus 

 ranged throughout the Chalk, but is abundant only 

 at restricted horizons. 



Nucleolitoida, which show much greater advances 



