FOSSIL EOHINODERMS. 125 



larger proportion of species are tropical and subtropical. 

 Mr. A. Agassiz divides the Eehinid fauna of the world into 

 four realms : the American, Atlantic, Indo-Pacific, and 

 Australian. 



Though Crinoids were the predominant type of Echino- 

 derms m the palseozoic rocks, a few star-fish and Ophiurans 

 appeared in the Upper Silurian period, and with them were 

 associated one species of sea-urchin, PalcRcliinus, though 

 the genus was more numerously represented in the Coal 

 period. Some Palseozoic forms resembled the living gen- 

 era Calveria and Phormosoma, and belong to the extinct 

 Carboniferous genera LepidecJiinus and Lepidesthes ; in all 

 these forms, fossil and recent, the interambulacral plates 

 overlapped one another so as to give a certain amount of 

 flexibility to the shell. This feature existed in a less de- 

 gree in Archmocidaris. The characteristic American car- 

 boniferous genera are Melonites, OUgoporus, and LepidecJii- 

 nus. The Permian Eocidaris is nearly allied to Archmoci- 

 daris, so that it is a true palseozoic type (Nicholson). 



In the Mesozoic epoch (Trias, Lias, and Jura) appeared a 

 more modern assemblage of Spatangidm, and genera such as 

 Hemicidaris and Hypodiadema, closely allied to the Cida- 

 ridcB proper, appeared in the Trias. The Jurassic beds are 

 characterized by genera allied to Diadema, Echinus, Ci- 

 daris, and a number of species of the families CassiduUdce 

 and Oaleritidm. A large number of genera survived in the 

 Cretaceous period, which, however, is characterized by the 

 marked development of the Spatangidcs. In the Upper 

 Cretaceous the earliest Glypeastridw appeared, while the 

 Tertiary Eehinid fauna is quite similar to the present one. 

 The striking fact in the geological history of the class is 

 the persistence of many of the cretaceous genera in the 

 abyssal or deep-sea fauna of the present time (A. Agassiz). 



