ECHINODERMATA. 13 



ruption from the mouth to the disc ; they arc 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 all represent generic and specific characters 

 of greater or less value, which require to be carefully noted in the description of the species. 



The Apical or Genital Disc. 



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

 most of the genera, of ten plates, — namely, five genital plates, and five ocular plates. (PI. Ill, 

 figs. 1 e, 2/, and PI. IV, fig. 2/.) In the Salenida there is one or more additional plates 

 introduced. The Jive genital or oviductal plates correspond to the summits of the inter- 

 ambulacral areas (PI. Ill, figs. 1 a, 3 a) ; 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. IV, figs. \g, 2/) is placed a spongy prominent mass, called the 

 madreporiform body. The plate, supporting this body, was supposed by Agassiz and 

 Desor always to represent the posterior part of the test, but we shall show in the sequel 

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



The ocular plates are placed at the summit of the ambulacral areas. They are small 

 heart-shaped bodies (PI. IV, fig. \g), and are wedged into the angles of the genital plates 

 around the circumference of the disc. 



The sur-anal plates are found only in the Salenidce. They consist of one or many 

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

 opening. (PI. XVI.) 



The anal plates are very small bodies, and are variable as to size and number ; they 

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

 seldom preserved in fossil species. 



The Tubercles. 



The plates which compose the test of the Cidarida, Echinida, and Salenidce, have large 

 tubercles developed on their external surface. They are divided into primary tubercles, 

 semi-tubercles, secondary tubercles, minute tubercles, granules, and miliary granulation. In 

 the other families the tubercles are smaller, more numerous, and less complicated. 



The primary tubercles form two rows in the inter-ambulacral areas of the Cidarida, 

 (Pis. I and II), and in the genus Hemicidaris (Pis. Ill and IV), and a section of the genus 

 Diadema (Pis. VI and VII) ; and four, six, eight, or ten rows in many Ecldnidce. 



The semi-tubercles are found at the base of the ambulacral areas of the genus Hemi- 

 cidaris. (PI. Ill, fig. 1 b, c ; PI. IV, fig. 1 c, d.) In a section of the genus Hemipedina, 

 they likewise are found at the base of the ambulacra (PI. XI, fig. 2). 



