235 
however, point out that I do not feel sure of the point, from which 
the Clypeastroids should proceed. Likewise I quite realize that the 
derivation of the Stirodonta from the Cidaroida, independently of 
the Diadematoids, involves some difficulties. In a letter to me 
Jackson justly points out that on assuming the phylogenetic inter- 
Amphisternata 
Camarodonta Meridosternata 
Cassiduloidea ) 
Cidareida i Clypeastroidea 
Å 
Å (2) $ 
É £, Holectypoidea 
Stirodonta 
Fa 
/ 
TØ 
14 
så Diadematoidea 
i (?) 
Fa 
Øg 
i 
Æ. Echinothuridæ 
i 
i Palæechinidæ Lepidesthidæ 
Årchæocidaridæ Haner 
ie ER 
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BR oe priser se 
sn me 
Bolleiuidkteld 
relations of the different groups as expressed in the above diagram 
quite a number of characteristic features must be accounted for 
as having developed independently in both groups. Jackson 
mentions as such: the two columns of plates in the interambulacra; 
only 10 primordial ambulacral plates on the peristone; the compound 
ambulacral plates; the peristomial gills, pits in the top of the 
