TERMINOLOGY. 25 



The pores are round, oblong, or elongated ; the pores forming a pair may be equal or 

 unequal ; in relation to each other, they may be transverse or oblique, contiguous or 

 remote, and when united by a transverse sulcus they are said to be conjugate. 



The pores are differently arranged in the zones in the different families : when they are 

 disposed in single pairs, they are said to be unigeminal (PI. VI, fig. 1, d); when in double 

 pairs, bigeminal ; when in triple oblique pairs, trigeminal ; and when grouped in a greater 

 number, as in many living species of the genus Echinus, they are polygeminal. 



When the zones extend in a straight uninterrupted line from the mouth to the apical 

 disc, they are said to be simple, as in the Cidarida, Echinida, Salenida, Galerilida, 

 Echinonida? ; when the zones, after parting from the apical disc, expand, and again 

 contract, thereby forming a leaf-like figure on the upper surface of the test, they are said 

 to be petaloidal, as in the Clgpeasterida ; when the petal is not so complete, as in the 

 Cassidulida, it is subpetaloidal. The zones are complete when they extend without inter- 

 ruption from the mouth to the disc ; they are interrupted when they terminate on the 

 upper surface, and reappear again at the base near the mouth ; they are limited when 

 they form only a star on the dorsal surface. These terms represent generic characters 

 of greater or less value, and require to be carefully noted in the description of the 

 species. 



The Apical or Genital Disc. 



The apical disc occupies the centre of the summit of the test, and is composed in 

 most genera of ten plates, namely, five genital plates and five ocular plates (PI. VI, 

 fig. 1, a). In the Salenida there is one or more additional plates introduced. The five 

 genital or oviductal plates correspond to the summits of the inter-ambulacral aveas ; two 

 plates form an antero-lateral pair, two a postero-lateral pair, and the single plate is 

 placed behind. On the right antero-lateral genital plate (PI. VI, fig. 1, a) is a spongy, 

 prominent mass, called the madreporiform bodg ; the plate supporting this body was 

 supposed by Agassiz and Desor always to represent the posterior part of the test, but I 

 have shown that it is invariably placed on the right antero-lateral plate. 



The ocular plates are at the summit of the ambulacral areas; they are small, heart- 

 shaped bodies (PI. VIII, fig. 4, b), wedged into the angles of the genital plates around 

 the circumference of the disc. 



The sur anal plates are found only in the Salenid^e; they consist of one or many 

 elements placed in the centre of the genital circle, and always before the anal 

 opening. 



The anal plates (PI. VIII, fig. 4, b), are very small bodies, and variable as to number ; 

 they clothe the membrane of the anal opening, and are well seen in recent urchins ; but 



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