GEOLOGICAL HISTORY OF SEA-URCHINS. 489 



disc has never more than four or five perforated plates. The Palae- 

 echinoidea comprise the Cystocidaris, formerly described as Echino- 

 cystites by Wyville Thomson. It is found in the Silurian rocks. 

 Bothriocidaris is a genus from the Cambrian of Russia, in which there 

 are only fifteen rows of plates in the test, one interambulacral row 

 dividing four rows of ambulacral plates. 

 The Perischoechinidce are distinguished by 

 having more than two rows of plates in each 

 interambulacral area. The group is almost 

 entirely of Carboniferous age, and includes 

 such genera as Lepidocentrus and Xeno- 

 cidaris from the Devonian, Pholidocidaris, 

 Perischodomus, Rhoechinus, Palcechinus, 

 which has a Silurian species ; Me/onites, Fi s- io2.-Patecfti*. 



Oligoporus, Protoechinus, Archceocidaris, 



Lepidocidaris, Lepidecliinus are common to Carboniferous and Devo- 

 nian, and Eocidaris is Permian. 



The ordinary sea-eggs are divided into the regular and irregular 

 types. The regular forms, usually termed " Endocyclica," commence 

 with the Secondary rocks. Cidaris dates from the Trias. This genus 

 has been subdivided into many sections, some of which have a 

 pala3ontological value. Thus Rhabdocidaris is characteristic of the 

 secondary rocks ; Diplocidaris and Polycidaris belong to the Oolites. 

 TemnocMaris is found in the Upper Chalk. Echinothuria is a flexible 

 sea-urchin, in which the plates overlap like those of the oral disc of 

 a Cidaris. It is only known from the Chalk, and is represented at 

 the present day by Phormosoma and Asthenosoma, or Calvaria. The 

 Salenia type commences in the Lias with Acrosalenia, which survives 

 to the Neocomian. It becomes accompanied in the Upper Oolites 

 by Pseudosalenia and in the Upper Greensand by Goniophoi^us and 

 Heterosalenia, and the still surviving genera Salenia and Peltastes. 

 The great family of the Diadematida3 commences in the Trias with 

 Hypodiadema, is succeeded in the Lias by Diademopsis and Micro- 

 diadema. In the Lower Oolites appear Pseudodiadema, which ranges 

 to the Chalk, and Hemipediita, which ranges from the Oolites to the 

 Neocomian. Pedina, Hemipygus, Glypticus, Acropeltis are character- 

 istic of the Upper Oolites. Hemicidaris and Acrocidaris both range 

 to the Neocomian, and Hemicidaris is also found in the Chalk ; under 

 the name of Hypodiadema it is known from the Permian and Trias. 

 The Neocomian genera, which are common to the Jurassic rocks, in- 

 clude Magnosia and Pseudocidaris. Cyplwsoma is common to the 

 Neocomian and Cretaceous ; and one species is found in Japanese 

 seas. Among cretaceous genera are Codtopsis, Echinocyphus, Leio- 

 soma, Leiocyphus, Glyphocyphus, and Heterodiadema. The Tertiary 

 rocks yield several genera, among which Ecliinopsis and Hebertia are 

 in the Lower Tertiary, and others, like Temnechinus, Temnopleurus, 

 and the Cretaceous genus Cottaldia, survive to existing seas. Coelo- 

 pleurus is a Lower Tertiary type, which still survives. 



The Echinidse appear in the Lower Oolites with Ecliinodiadema, 



