221 
One of the main characters upon which my classification was 
founded, is the structure of the ambulacra, whether simple or com- 
pound, and in the latter case, whether of the diadematoid or the 
echinoid type (viz. in the former the second of the primary plates, 
in the latter the first of the composing primary plates being the 
largest). I think Jackson is right in not regarding the distinction 
between those with simple and those with compound plates as of 
essential importance, from which it follows that the Saleniidæ must 
be united with the Arbaciidæ etc. The difference hetween the two 
types of compound ambulacra, on the other hand, I would not be 
ineclined to drop as a character of primary importance. Jackson 
does not mention this feature at all and appears to have paid no 
attention to it, no reason being given for thus disregarding a 
structural feature to which other workers (Duncan and myself) 
have ascribed a considerable systematic value. This accounts for 
the different place given to the Stomopneustidæ. According to the 
structure of the ambulacra it belongs to the Camarodonta, according 
to the character of the ”"foramen magnum” it belongs to the Stiro- 
donta. Here is a contradiction between two important characters; 
for the present it is impossible to settle the question. The only 
certain thing is that Stomopneustes represents a very peculiar type, 
not very closely allied to any of the other Diadematoids. 
While the classificatory results of Jackson's researches on 
the Diadematoids are thus mainly a confirmation of my views on 
their mutual interrelations, the names Au/odonta, Stirodonta and 
Camarodonta are as new as they are excellent. They could scarcely 
be better and will certainly be generally adopted. — I am sorry 
that I cannot express the same satisfaction with the main result 
of the whole work: the classification of the Echinoidea and the 
views on their phylogeny as expressed therein. 
It has been rather universally adopted in recent times that 
the Palæozoic Echinoids should not be regarded as a separate 
group, in opposition to all the rest of the Echinoidea. Though 
Jackson does not directly maintain the old arrangement, his 
