300 Agassiz—Paleontological and Embryological Development. 
zone of the Hemicidaride, next into the indistinct ares of pores 
of the Pseudodiadematide, then into the ares with a limited 
number of pores of the Triplechinide, and finally to the polypo- 
rous ares of the Echinometrade. hat can be more direct 
than the gradual modification to be traced in the development 
of the primary ambulacral tubercles, such as are characteristic 
arid primary ambulacral tubercles, and an Nchinid_por- 
iferous zone. In the same way in the Diadematide, the large 
primary interambulacral tubercles are Cidaridian features, 
while the structure of the ambulacral tubercles is Hemicidarid- 
ian. The existence of two kinds of spines is another Cidarid- 
ian feature, while the apical and actinal systems have become 
modified in the same direction as that of the Echinide. The 
combination in time, may often reveal the elements from which 
have been produced apparently unintelligible modifications. 
