THE BLASTOIDEA. 509 



of extinct Echinoderms {Eclrioaster, Agelacrinites, Semicys- 

 tites), which, in general form, somewhat resemble what the 

 Asterid Gonmster would be if its angles w^ere rounded ofP. 

 Like the Cystldea^ they possess an interambulacral^9yra/?ziV^, 

 but they differ from them in that they have ambulacra per- 

 forated by canals which open directly into the cavity of the 

 calyx, and that they possess no arms. The Edrioasterida 

 have no stem, but seem to have been attached by the abo- 

 ral face of the body. 



The Blastoidea. — In Pentremites^ the representative of 

 this order, the ambulacral and antambulacral regions are 

 nearly on an equality : the body is prismatic or subcylin- 

 drical. Tlie pedunculated calyx is composed of three basal 

 plates, two of which are double. The aboral plates receive 

 in their intervals five plates deeply cleft above. In the clefts 

 lie the apices of the ambulacra, the oral portions of which are 

 included between the five deltoid interradial pieces which 

 surround the mouth. The cleft plates are not radials, but 

 portions of the perisomatic skeleton of the aboral region. 

 Surrounding the central, probably oral, aperture, are four 

 double pores, and a fifth divided into three. The median of 

 these three seems to be anal, the others and the paired pores 

 being genital. Each ambulacrum is lanceolate in form, and 

 presents superficially a double row of ossicles, which meet in 

 the middle line and support pinnules at their outer extremi- 

 ties ; beneath them lies a single plate, perhaps the homologue 

 of the vertebral ossicles in the Ophiurldea / beneath it again 

 are parallel canals, the nature of which is unknown. 



