METAZOA ECHINODERMA. 175 



The number of rows of plates in the interambulacral 

 areas of the simplest species of this genus is four, and of 

 the most specialized eight. 



The adult Palaeechinus gigas McCoy, has the primitive 

 ambulacral plates, a, b, on the margin of the ambulacral 

 area (PI. 318, fig. 2, shows a portion of one ambulacrum ; 

 b, primitive plate ; a not drawn), while two other plates 

 have arisen (a 1 , b 1 }. The interambulacrum has from five 

 to six or seven rows of plates ; six are clearly shown by 

 the red dotted lines in fig. 3. This drawing begins at the 

 point of origination of the fifth row of plates, those below 

 this point not being preserved in the specimen. The fig- 

 ure shows that the columns 5 and 6 originate in a single 

 plate as we have already seen is the case with the inter- 

 ambulacral rows of Bothriocidaris. The initial plate is 

 always near a seven-sided or heptagon plate (fig. 3, If). 

 In Palaeechinus the anal disc is surrounded by the ring 

 of alternating genital and ocular plates ; the former are 

 pierced by three holes while the latter have two. 



When we pass to the genus Oligoporus we find in the 

 young as represented at the ventral border two rows of 

 ambulacral plates (PI. 319, fig. i, a, b) , while farther up 

 the corona the adult condition of four plates (fig. i, a, ft, 

 a', // ; fig. 2) is seen, and still farther up new plates arise 



(% 2). 



The number of rows of interambulacral plates has in- 

 creased to nine (fig. 3) in the most specialized species of 

 this genus (^Oligoporus danae M. & W.). 



The forms we have already described lead the way to a 

 better understanding of the complex structure of Melo- 

 nites (No. 320, a-d, M. multiporus Norw. & Owen). 

 These fossils are occasionally preserved with some of the 

 spines attached. The latter are small (PI. 321, fig. i, 

 magnified 6 + diameters) and when not fastened are some- 

 times found in the hollows of the corona. 



The ventral border of the shell (see No. 320, a, b\ PI. 

 321, fig. 2) shows the ambulacra and interambulacra. 



