PIIORMOSOMA TIISTTDUM. 



09 



of the auricles (PI. 45, figs. -^, 6). Seen from the exterior the crowding 

 of the right anterior ambulacral plates is .^hown on PI. 40, fig, (;, and the 

 corresponding view of the actinal plates of the left posterior ambulacrum 

 on PI. 45, fig. 7. 



The auricles first appear on the fourth plate (PI. 44^ fig. /); tlie open gap 

 between them has become closed in a specimen of 4o mm. (PI. 44, fig. J), in 

 which the auricles are now on the sixth pair. In a specimen of 64 nun. 

 they are partly on the seventh and partly on the eighth pair, and in a 



specimen of 137 mm. (PI. 44, fig. ^) they are on the ninth pair, except in 

 the anterior zone of the right anterior ambulacrum, in the right zone of the 

 odd ambulacrum, and in the posterior zone of the left posterior ambulacrum, 

 where they are on the eighth pair. The movement of the ambulacral coronal 

 plates cannot be better illustrated than by this difference, due to growth, 

 in the position of the auricles, and by the difference in the size of the plates 

 in front of and behind the auricles. From three to four plates of each 

 ambulacrum have slipped under the auricle thus 

 forming its component parts. 



When passing onto the actinostome under 

 or by the auricles, the inner part of some of the 

 larger ambulacral plates is left behind ; thus a 

 plate without pores remains in the ambulacral 

 series ; it is marked with a cross on Figs. 

 141-144, taken from specimens of 53, 64, and 



203 mm. in diameter, .see also Fig. 129. From the part of that plate 

 which borders the interambulacrum rises a ridge extending to the adjoin- 

 ing plates and uniting with the auricles. Fig. 144. 



Fig. 144. P»ORMOSf)MA HISPIDIM. 



