235 
bowever, point out that I do not feel sure of the point, from whioh 
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 
Jacks on justly points out that on assuming the phylogenetic inter- 
Camarodonta 
Cassiduloidea 
V / 
Cidaroida \ 
I \ / 
\ / 
V / 
Stirodonta 
Amphisternata 
Meridosternata 
Clypeastroidea 
Holectypoidea 
Diadematoidea 
Echinothuridæ 
Bothriocidaroida 
lelations 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 pnmordial ambulacral plates on the peristone; the compound 
ambulacral plates; the peristomial gilis, pits in the top of the 
