188 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The large "forked plates" correspond to the radials of tlic crinoids (including 

 the axilhirics, wliich niorjihologically are redui)licated radials), and to the 10 

 so-called ambulacrals wliich are the iirst to be formed in the echinoids. 



The plates mthin the central furrows of the forked plates correspond to the 

 bracliials of the crinoids (except the first two), and to the auricles and plates of the 

 dental pyramids of the urchins. 



The five jilates about the ventral apex correspond to the orals of the crinoids and 

 have no counterparts in the urcMns. 



The blastoids resemble the oclunoids in having the ambulacral structures 

 drawn out into five long narrow hnes extending toward the apical pole and covered 

 by a double row of similar small jilates, which, however, are not in any way homol- 

 ogous with the ])lates of the echinoids wluch occur in the same situation. 



In very small specimens the forked plates scarcely differ in shape from the 

 tji^ical crmoid radials, there being merely a sUght concavity in the distal border. 

 The (central portion of the plate ceases to extend itself ventraUy, but the sides become 

 enormously ])roduced, inclosing the ambulacrals as they are formed. 



The forked ])late represents the crmoid radial and the entire series of so-called 

 ambulacrals of tlie ccliinoid. The first two am])ulacrals formed in the conca%'ity 

 on its distal edge, lying side by side, are therefore identical in position ^vith the 

 auiicles of the echinoids, and form a circlet of 10 plates arranged in pairs just 

 beyond the radials (or ambulacral series). Instead of being wholly internal like 

 the auricles, or of extending themselves outward and away from the body hke the 

 crinioid brachials, these plates lie in the body wall flush with the forked plates, just 

 as do the entirely different echinoid ambulacrals. 



In the echinoids the radial processes from the various circumoral systems are 

 more or less attached to the distal portion of the ocular plates; with the growth of 

 the test these radial ambulacral processes become drawn out, and are continually 

 being covered, as necessity requires, by a continuous formation of new plates at the 

 distal border of the oculare. The first two plates formed (comparable to the forked 

 ])late of the blastoids and to the radial of the crinoids) always maintain their original 

 position on the edge of the peristome, with the circlet of auricles and dependent 

 plates just within them. 



In the blastoids the ocular plates are absent, and the radial processes from 

 the various circumoral systems are attached to the distal portion of the radial 

 plate instead. But this amounts to the same thing, for in both cases these proc- 

 esses are attached to the distal border of the first radially situated plate. As the 

 animal grows the ambulacral processes are drawn backward down the sides 

 exactly as in the echinoids. 



The forked plate represents the entire ambulacral series of the echinoids, and 

 the radials, including the axiUaries, of the crinoids; on its distal border are two 

 little jjlates similar to the auricles of the echinoids. Now the auricles of the 

 eclunoids may be elongated by the addition of new plates to theii- distal (ventral) 

 ends; sinularly in the blastoids the small plates within the concavity of the distal 

 border of the radials, on drawing away from the ventral apex of the animal, con- 



