BIOLOGICAL PALAEONTOLOGY 99 



to interambulacral plates of the species. Above the 

 mid-zone, the characters of the plates show steady 

 simplification as they are followed towards the apical 

 system, attaining eventually a morphogenetic phase 

 hardly to be distinguished from that of the first post- 

 larval plates. 



The corona of an Echinoid thus presents two some- 

 what similar (though reversed) sequences of morpho- 

 genesis. The plates below (adoral to) the mid-zone 

 show, with certain modifications, the actual stages of 

 morphogeny as they progressed in company with onto- 

 geny ; those above that zone show morphogenetic stages 

 that are independent, in some measure, of the growth of 

 the individual. But the ontogenetically-late "young 

 plates " rarely follow exactly the path of development 

 taken by their forerunners. The adult or senescent state 

 of the organism induces various accelerations and modi- 

 fications in the character of the new plates ; however, 

 these features rarely produce complications greater than 

 those normal to ontogenetic stages. Perhaps the most 

 satisfactory quality of this " continuous " type of growth 

 is its evidence for the phyletic stage of the species in 

 which it occurs. In Plesiechinus^ as in Regular Echi- 

 noids, increase in length of the columns is achieved by 

 accretion of new plates at one end (the apicad) only. 

 Hence the horizon of the " mid-zone " steadily rises 

 during life. This means that each one of the plates 

 between the mid-zone and the peristome has been, in 

 earlier stages of ontogeny, a " mid-zonal " plate. Apart 

 from minor modifications, it retains in perpetuity the 

 " finished " state of morphogeny associated with the 

 ontogenetic stage when it was at the mid-zone. It 

 follows, then, that if the mid-zonal plates of an adult 

 specimen are the most highly specialized in the corona, 

 the species to which it belongs is either progressive or at 



