THE AMBULACB \. 



clearly seen in a large specimen of Pentremifi is sulcatus from the Chester limestone 

 in which the peristome is covered by a somewhat extensively developed vault (PI. I. 

 fig. 8). The re-entering angles of the vault correspond to the median grooves of the 

 ambulacra, which are completely covered hy a double series of alternating plates that 

 interlock over the middle of the groove so as to close it entirely. They are much 

 more regular in their arrangement than we have seen in any other Blastoid, and we 

 have no hesitation in regarding them as homologous with the alternating radial 

 dome-plates of Cyathocrinus and of the Platycrinidce, and with the covering plates 

 of the ambulacra in recent Crinoids. They have the same relation to the more or 

 less regular "apical dome-plates " which cover the peristome of the Blastoids as the 

 radial dome-plates of Platycrinus bear to the proximals, and the covering plates of 

 the disc-ambulacra in the recent Hyocrinus or Rhizocrinus to the oral plates. 



