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ECHINOIDEA. 



Taking these general principles for ray guidance, I subdivide the Echinoidea into 

 the following thirteen natural families -. 



Order— ECHINOIDEA. 



Echinoidea endocyclica. 



Vent within the genital plates, 

 always opposite the mouth. 



Echinoidea exocyclica. 



Vent without the genital plates, 

 never opposite the mouth. 



Cidaridae. 



Hemicidaridae. 



Diademadae. 



Echinidae. 



Salenidae. 



Echinoconidae. 



Collyritidae. 



Echinonidae. 



Echinobrissidae. 



Echinolampidae. 



Clypeasteridae. 



Echinocoridae. 



Spatangidae. 



Family 1. CiDARiDiE. — Test thick, spheroidal; inter-arabulacral areas very wide; 

 primary tubercles large, perforated ; bosses crenulated or uncrenulated, spines large, 

 thick, mostly claviform ; ambulacral areas very narrow ; poriferous zones narrow, pores 

 unigeminal, rarely bigeminal ; mouth-opening large, inferior, central, circular or penta- 

 gonal ; peristome destitute of notches, always armed with large, powerful jaws. Vent- 

 opening wide, superior, opposite the mouth, surrounded by five large genital plates 

 perforated for the ovarial canals, and five small ocular plates excavated for lodging the 

 eyes ; buccal and anal membranes covered with scales. 



Types. Cidaris claviyera, Konig (PI. IV). Cidaris sceptrifera, Mantell (PL VI). 



Family 2. Hemicidarid^;. — Test thick, spheroidal, or more or less depressed ; ambu- 

 lacral areas narrow or wide, with semi-tubercles at their base only, or extended 'throughout 

 the area ; inter-ambulacral areas with two rows of primary tubercles, rarely more than 

 eight in each row ; tubercles of both areas perforated, and bosses deeply crenulated ; 

 poriferous zones narrow and undulated ; pores unigeminal throughout, except near 

 the peristome, where they are bigeminal and trigeminal. Mouth-opening large ; peristome 

 decagonal, and divided more or less deeply by notches into ten lobes ; jaws large and 

 powerful ; apical disc small, opposite the mouth, composed of five genital and five 

 ocular plates. Spines long, thick, cylindrical, tapering, claviform, or stout, compressed, 

 or angular; surface smooth, or covered with fine longitudinal lines, as far as known, 

 neither prickles nor asperities are developed thereon. 



Types. Hemicidaris intermedia, Fleming. Acrocidaris formosa, Agassiz. 



Family 3. Diademadae. — Test thin, circular, or pentagonal, more or less depressed ; 



