METAZOA ECHINODERMA. 183 



vertical rows or in arcs. The internal partitions so char- 

 acteristic of the irregular sea urchins are found ^n the 

 adult, sometimes, however, in a rudimentary condition. 

 The typical characteristics of the first division of the 

 irregular sea urchins are well shown in Clypeaster sub- 

 depressus Ag. (No. 337), and for this reason, it would 

 seem, the name of Clypeastroids is given to the division. 

 There is, however, no well denned line between this 

 group and the next, the Spatangoids, so that it seems 

 better to place both under the head of the irregular sea 

 urchins. 



The corona of Clypeaster is flattened and longer than 

 it is broad. It can be so placed as to bring the odd am- 

 bulacral petal in the median line, and the remaining two 

 pairs of petals on either side, thus dividing the test into 

 two nearly equal parts. 



The mouth is near the middle of the lower side and the 

 simple ambulacral grooves extend outward from it. There 

 are delicate spines in the grooves, and stronger ones on 

 top of the conspicuous petals when these are formed. 



The anal system has moved from the upper to the 

 lower side, near the margin, and the anus is seen in No. 

 337, so that the Clypeastroid has an anterior and a pos- 

 terior end. 



The abactinal area is far more indefinite than in the 

 regular urchins. It is made up partly of the madreporic 

 body which is in the center. Four or five genital open- 

 ings are seen, but the plates themselves do not appear. 



If the upper portion of the test is removed (No. 337), 

 the immense jaws with two teeth are exposed. These 

 jaws consist of five strong parts which taken together con- 

 stitute a powerful eating apparatus. The upper and lower 

 parts of the shell are connected by slender pillars (No. 

 337) which are an important characteristic of the Clype- 

 astroids, the regular sea urchins having nothing of the 

 kind. Around the outer edge these pillars unite, forming 

 more or less open walls, as seen in No. 337 where a por- 

 tion of the edge has been removed. 



