Schizaster compactus from Western Australia 
323 
The characterisation of species of Schizaster, and 
of echinoid species in general, needs to take into 
account both ontogenetic and general adult 
variation. The ontogenetic variation arises either 
from differentiation of novel features during 
growth (such as pore pairs or tubercles) or from 
growth of particular structures, such as coronal 
plates (McNamara 1988). Allometric growth of 
particular plates, either positive or negative, 
characterise species, but small variations to the 
allometries are the factors that produce adult 
phenotypic variation. The degree of variation itself 
can be a feature that characterises a species. High 
degrees of variation certainly make it harder to 
characterise species, but it is vital that both 
ontogenetic and adult phenotypic variation be 
taken into account when characterising species, 
either living or fossil. 
ACKNOWLDEGEMENTS 
I am grateful to Harry Butler and Bob Nunn for 
bringing the specimens to my attention. I wish to 
thank Kris Brimmell for the photographs. Thanks 
to Steve Donovan and Burt Carter for reviewing 
the manuscript and making helpful suggestions 
that tightened up the paper. 
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Manuscript received 28 February 1995; accepted 5 May 1995. 
