ECHINODERMATA. BLASTOIDEA. 



95 



the following structure. The dorsal part is formed of a 

 cycle of three plates the basals (6), two of which are 

 alike, and the third smaller. Above the basals, comes a 

 cycle of five radial plates (r), these are larger than the 

 basals and form the main part of the calyx. At the upper 

 end of each there is a deep incision, which serves for the 

 reception of the ambulacral area (a). Above the radials 

 and alternating with them comes a cycle of smaller plates 

 the deltoids (d). The mouth is placed at the summit 

 of the calyx, in the centre, and around it are five other 

 apertures termed spiracles (s), one of which is larger than 

 the others and includes the anus (an). From the summit 

 the five ambulacral areas (a) radiate towards the dorsal 

 surface, they are bordered partly by the deltoids but 

 mainly by the radials. Each ambulacral area (fig. 25) 



FIG. 25. Ambulacral area of Pentremites godoni, Carboniferous. I, lancet 

 plate ; s, side plate ; p, pore ; a, ambulacral groove ; sp, spiracle. 

 (Three times natural size.) 



consists of the following plates. Occupying the middle is 

 a long pointed plate (I), the lancet plate, which is traversed 

 by a longitudinal canal, in which the radial vessel of the 



