FOSSIL ECHINODEHMS. 125 



larger proportion of species are tropical and subtropical. 

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

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

 Australian. 



Though Crinoids were the predominant type of Echino- 

 derms in the palaeozoic rocks, a few star-fish and Ophiurans 

 appeared in the Upper Silurian period, and with them were 

 associated one species of sea-urchin, Palceckinus, though 

 the genus was more numerously represented in the Coal 

 period. Some Palaeozoic forms resembled the living gen- 

 era Calveria and Pltormosomn, and belong to the extinct 

 Carboniferous genera Lcp'dccli>nun and Lepidesthes j 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 ArchcBoddaris. The characteristic American car- 

 boniferous genera are Melonites, OUgoporus, and Lepideclii- 

 mis. The Permian Eocidaris is nearly allied to ArcJueoci- 

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



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

 more modern assemblage of Spatangidce, and genera such as 

 Hemiciduris and Hypodiademn, closely allied to the Cida- 

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

 characterized by genera allied to Diadema, Ecliinus, Ci- 

 daris, and a number of species of the families CassidulidcB 

 and Galeritidce. A large number of genera survived in the 

 Cretaceous period, which, however, is characterized by the 

 marked development of the Spatangi.dce. In the Upper 

 Cretaceous the earliest Clype<istrid<.e appeared, w r hile the 

 Tertiary Echinid 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). 



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