186 SYNOPTIC COLLECTION. 



r, s). Around the madreporic body are five ocular 

 openings at the ends of the ambulacra, and four genital 

 openings at the ends of the interambulacra. The inter- 

 ambulacrum opposite the odd petal is without a pore. 

 The preparation (No. 344) shows the five pairs of parti- 

 tions which radiate from the edge towards the center 

 with space between each pair of partitions. There are 

 other pairs that occupy the interambulacral areas while 

 many pillars are crowded together on the ambulacra. 



Greater specialization of structure marks the species 

 Echinarachnius excentricus Val. (No. 345). Here the 

 lower side is marked by radiating furrows that divide 

 close to the mouth and afterward subdivide and send their 

 branches over to the upper surface. Three of the petals 

 are larger than the other two, and the mound bearing the 

 rosette is not in the center but nearer the posterior end. 

 The anus has moved from the edge to the lower side. 



Encope grandis Ag., when young is circular in outline 

 and is without the rosette or lunules. The adult (No. 

 346) has five large openings into the margin besides the 

 completed lunule in the median interambulacrum. The 

 petals of the rosette differ in size and shape, the pos- 

 terior pair being longer than the others and extending 

 nearly to the lunules. 



The madreporic body is star-shaped and four genital 

 openings are at the tips of the rays, while the fifth 

 opening is nearer the center. The five ocular pores are 

 at the angles of the rays. 



In conclusion it may be said that the young of all the 

 Clypeastroids are much more like Echinometra and the 

 regular sea urchins than they are like the adults of their 

 own group. This is sufficient reason for placing the 

 Clypeastroids next the regular sea urchins and before the 

 Spatangoids. 



