ECHINOIDEA. 239 



are on the posterior inter-radius, interambulacra multiserial, Silurian. 

 Palaeodisctis Salter, with flattened body, Silurian. Lepidocentrus J. 

 Mull., Devonian. Koninckocidaris Dollo and Buis., Carboniferous. 

 Perischodomus * McCoy, Garb. Archaeocidaris McCoy, Carb. Lepido- 

 cidaris Meek and Worthen, Carb. Lepidechinus Hall, Dev., Carb. Palae- 

 echinus McCoy, Sil., Carb. Rhoechinus W. Keeping, Carb. Melonites 

 Norwood and Owen, Carb. Oligoporus Meek and Worthen, Carb. Lepi- 

 desthes Meek and Worthen, Carb. Tiarechinus Neumayr, with only four 

 plates in each interambulacrum, one at the peristomlum and three 

 extending side by side from the peristomial to the apical system ; apical 

 system unusually large. 



Order 2. ETJECHINOIDEA f 



With two vertical rows of plates in each of the five interradii, 

 and a similar number of vertical rows of simple or compound plates 

 in each of the five radii. 



The peristome is on the oral side and rarely placed anteriorly 

 towards the edge of the shell. Jaws and teeth are present or 

 absent. The anus is either within the apical system, or in the 

 posterior interradius. The sub-class comprises some extinct 

 and all recent forms. The Cidaroida and Diadematoida are 

 Endocyclica or regular sea-urchins, the Holectypoida, Clypeas- 

 troida and Spa.tangoida are Ectocyclica or irregular forms. 



Sub-Order 1. CIDAROIDA. 



Test spheroidal. Ambulacra narrow, usually composed of primary 

 plates, rarely compound. Mouth central, anus within the apical system. 

 With internal branchiae only (Endobranchiata). With jaws and more or 

 less vertically placed teeth and a discontinuous perignathic girdle. With 

 large spines and tubercles. The interradial as well as the ambulacral 

 plates are continued on to the oral area to the mouth, and are imbricated 

 onHhe peristome. Sphaeridia, ophicephalous and triphyllous pedicellariae 

 are absent. Carboniferous to the present day ; principal distribution in 

 the Jurassic and Cretaceous. 



Fam. 1. Cidaridae with the characters of the order. Dorocidaris A. 

 Ag., N. part of Atlantic Ocean, 50-1,500 fms., D. papillata Leske, W. coast 

 Ireland. Cidaris Leske, cosmopolitan in the warm seas, littoral to 300 

 fms. Phyllacanthiis Brdt., Red Sea to Australia, littoral. Porocidaris 

 Desor, P. purpurata W. Th., N. Atl., 300-1,500 fms. 



Extinct genera : Orihocidaris Cotteau, Temnocidaris Cotteau, Diploci- 

 daris Desor, Tetracidaris Cotteau, Cretaceous, see note, p. 238. 



Sub-Order 2. DIADEMATOIDA. 



Mouth central, anus within the apical system. J Internal branchiae 

 well developed, reduced, or absent. With external branchiae (Ectobran- 



* Sollas, Quart. Journ. Geol. Soc., 55, 1899, p. 70. 



f P. Martin Duncan, op. cit. 



j In one extinct genus, Heterodiadema, the posterior basal is absent, and 

 the periproct is pushed back a slight distance into the posterior inter- 

 radius. 



