106 DIADEMADtE. 



Family 3 — DiADEMADiE. 



This family includes large and small urchins, with the test thin, circular, pentagonal, 

 or sub-pentagonal ; more or less depressed ; mouth opening large, central ; peristome 

 decagonal, and divided into ten lobes by deep notches. 



Apical disc small, directly opposite to the mouth, and composed of five genital, 

 and five ocular plates ; the anterior pair of genital plates are, in general, a little larger 

 than the posterior pair; and the right antero-lateral plate, with the small, spongy, madre- 

 poriform body on its surface, is the largest. The vent is round or oblong, and is generally 

 in the centre of the genital circle. 



The ambulacral areas are more or less wide ; sometimes they are one half the width 

 of the inter-ambulacral areas, and furnished with two or four rows of primary tubercles, 

 often as large and as numerous as those of the inter-ambulacral areas. 



The poriferous zones are narrow, and almost always straight ; the pores are unigeminal, 

 bigeminal, or trigeminal in their disposition in different genera. 



The inter-ambulacral areas are, in general, twice the width of the ambulacral ; there 

 are from eight to fourteen plates and upwards m each column ; the areas are occupied by 

 two, four, six, or eight rows of primary tubercles, nearly all of the same size ; or there are 

 rows of primary tubercles, with two or four rows of secondary tubercles, much smaller in 

 size, filling up the interspaces of the area. The bosses of the tubercles are small, and 

 their summits are in general crenulated, but sometimes they are uncrenulated; the spini- 

 gerous tubercles are small ; in general they are perforated, rarely are they imperforate ; 

 the presence or absence of these characters, added to the structure of the poriferous zones, 

 afford, when taken collectively, good generic characters. 



In general the inter-ambulacral tubercles are larger than the ambulacral, but sometimes 

 they are of equal sizes in both areas, a character which distinguishes the genera of this 

 family from those of the Cidakid^ and Hemicidaridje. 



The spines in the living genera are long, slender, and tubular ; sometimes they are 

 thrice the length of the diameter of the shell.* In the fossil genera they rarely attain the 

 length of the diameter of the shell, and are stout and solid ; the slender, tubular spines have 

 their surface ornamented with oblique annulations of fringe-like scales ; whilst the surface 

 of the solid fossil spines is covered with fine, longitudinal lines ; but neither prickles 

 nor other asperities are developed on their stems. 



Lamarckf subdivided Klein's genus Cidaris into two sections, the " Turbans" and the 



* Peters, Uber die Grnppe der Diademen, p. 7. 



f ' Histoire des Animaux sans Vert^bres,' torn, iii, 1st edit., pp. .')4— 58. 



