18 



NATURAL FAMILIES. 



Taking these general principles for our guidance, we subdivide the Echinoidea into 



the following natural families 



Order— ECHINOIDEA. 



Echinoidea endocyclica. 



Anus within the genital plates, 



always opposite the mouth. 



Echinoidea exocyclica. 



Anus without the genital plates, 

 never opposite the mouth. 



Cidaridse. 



Hemicidaridae. 



Diademadse. 



Echinidae. 



Salenidae. 



Echinoconidae. 



Collyritidse. 



Echinonidae. 



Echinobrissidae. 



Echinolarnpidae. 



Clypeasteridae. 



Echinocoridte. 



Spatangidae. 



Family 1. Cidarid^e. — Test thick, spheroidal; inter-ambulacral areas very wide ; pri- 

 mary tubercles large, perforated ; bosses crenulated or uncrenulated, spines large, thick, 

 and 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 and powerful jaws. Anal 

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

 for the genital canals, and five ocular plates excavated for lodging the eyes ; buccal and 

 anal membranes covered with scales. 



Types. Cidaris floriyemma, Phillips. Cidaris Sinithii, Wright. (PI. II.) 



Family 2. Hemicidaridae. — 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 ; the tubercles of both areas are perforated, and the bosses are deepl y 

 crenulated ; the poriferous zones are narrow and undulated ; the pores are unigeminal 

 throughout, except near the peristome, where they are bigeminal and trigeminal. The 

 mouth opening is large; the peristome is decagonal, and is divided by more or less deep 

 notches into ten lobes; the jaws are large and powerful; the apical disc opposite the 

 mouth is small, and is composed of five genital plates and five ocular plates. The spines 

 are long, thick, cylindrical, tapering, or claviform, or stout, compressed, and angular ; 

 their surface is smooth, or covered with fine longitudinal lines, but, as far as is known, 

 neither prickles nor asperities are developed thereon. 



Types. Hemicidaris intermedia, Fleming (PI. IV). Acrocidaris formosa, Agassiz. 



Family ."i. Diademadse. — Test thin, circular, or pentagonal, more or less depressed ; 



