188 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



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

 the axillaries, which morphologically are reduplicated radials), and to the 10 

 so-called ambulacrals which are the first to be formed in the echinoids. 



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

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

 dental pyramids of the urchins. 



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

 have no counterparts in the urchins. 



The blastoids resemble the echinoids in having the ambulacral structures 

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

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

 ogous with the plates of the echinoids which occur in the same situation. 



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

 typical crinoid radials, there being merely a slight concavity in the distal border. 

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

 enormously produced, inclosing the ambulacrals as they are formed. 



The forked plate represents the crinoid radial and the entire series of so-called 

 ambulacrals of the echinoid. The first two ambulacrals formed in the concavity 

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

 auricles 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 like 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 oculars. The first two plates formed (comparable to the forked 

 plate 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 echiuoids, and 

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

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

 echinoids may be elongated by the addition of new plates to their distal (ventral) 

 ends; similarly 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- 



