FOSSIL ECHINODERMS., 207 
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 paleozoic rocks, a few star-fish and Ophiurans 
appeared in the Upper Silurian period, and with them were 
associated one species of sea-urchin, Palechinus, though 
the genus was more numerously represented in the Coal 
period. Some Paleozoic forms resembled the living gen- 
era Calveria and Phormosoma, and belong to the extinct. 
Carboniferous genera Lepidechinus 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 Archeocidaris. The characteristic American car- 
boniferous genera are Melonites, Oligoporus, and Lepidechi- 
nus. The Permian Hocedaris is nearly allied to Archeoci- 
daris, so that it is a true paleozoic type (Nicholson). 
In the Mesozoic epoch (Trias, Lias, and Jura) appeared a 
more modern assemblage of Spatangide, and genera such as 
Hemicidaris and Hypodiadema, closely allied to the Cida- 
vide proper, appeared in the Trias. The Jurassic beds are 
vharacterized by genera allied to Diadema, Echinus, Ci- 
daris, and a number of species of the families Cassidulide 
and Galeritide. A large number of genera survived in the 
Cretaceous period, which, however, is characterized by the 
marked development of the Spatungide. In the Upper 
Cretaceous the earliest Clypeastride appeared, while 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 
abyssa! or dewp-sea fauna of the present time (A. Agassiz). 
