200 PROF. p. MARTIN DUNCAN ON THE 



behind by the lines just raentioried, are the interradial projec- 

 tions of the ambulacra! plates. Plates 4 and 5 will be seen 

 to have rounded and tall plate-ends which come up to the 

 ambulacral surface just below the position of the figures. 



But no other plates come up to the line, and they are all at 

 the base and within the mass of the process. The line marked a? 

 is of great interest, and in some specimens it is visible without 

 reagents on the peristomial face of the test (fig. 40). In this figure 

 the suture, for such is the inner part of the line, passes towards 

 the median line from the suture between the process and the 

 ridge. In fig. 39 the line of suture x passes to the peristome, and 

 it marks the upper surface of the poriferous zone whence the 

 process started. 



EcMnometra hicunter, Leske, sp. — The most striking part of 

 the perignathic girdle of the species of Echinometra is the cap, or 

 top projection of the combined processes (fig. 36). This cap is 

 moderately large in the species now under consideration ; and it 

 seems like a growth upon the tops of the processes, which covers 

 each one and joins it with its fellow. But the caps are not new- 

 growths, nor are they produced by any additional structures ; for 

 benzene fails to detect any divisional line between them and the 

 top and posterior part of the processes. The one structure 

 merges into the other, and the caps are growths of the ordinary 

 tissue of the processes. The direction of the processes is upwards 

 and backwards, so that their tops are much more distant from 

 the polar axis of the test than the peristome. The caps seem to 

 diminish this distance in E. lucunfer, and they evidently give 

 additional points of attachment to the retractor muscles. The 

 arrangement of the pores and their plates on and in the flank of 

 a process close to its base are very much as in Strongylocentrotus ; 

 and the suture between the process and the ridge fails to be in 

 contact with at least three ambulacral plates at the back of the 

 process. 



The ridges of the girdle are long and low ; and they are not 

 made after the type of those of the Echinidse or Temnopleuridae, 

 but after that of the allied genus Strongylocentrotus. 



Two conditions prevail, and in one (fig. 35) what may be called 

 the normal condition is seen; that is, there are two plates at the 

 edge of the ridge above, and one is the first plate of zone a, and 

 the other is the first plate of zone h, and they are symmetrical. 



The median line is short, for the plates are low. In zone a 



