234 
There still remain the Plesiocidaroida, which Jackson also 
regards as a separate branch which "with present knowledge is not 
closely affiliate with any other group” (p. 220). Jackson thus 
does not accept the highly interesting suggestion af Dåderlein 
(Echinoidea d. deutsch. Tiefsee-Exp. p. 183—184) of its relations 
to the Arbaciids, based mainly on his discovery that an unpaired 
median interambulaeral plate also exists in the recent Pygmæo- 
cidaris prionigera. He suggests (p. 215, note) that "it is possible 
that Dåderlein was misled in introducing the sutures of this 
plate, a point difficult to ascertain in such a small specimen 
(4,8 mm) as he had.” I do not see what right Jackson has to 
doubt the correctness of Dåderlein's definite statement, which 
gives so natural an explanation of the otherwise rather enigmatical 
Tiarechinus. It is true that Bather (Op. cit. p. 257—259) has 
given another explanation of this median plate and comes to the 
result that there is no mearer relation between Tiarechinus and 
Pygmæocidaris. But I am by no means convinced that Bather 
is right and am still inclined to accept Dåderlein's view. To 
enter in a more detailed way on this most interesting question is, 
however, not the place here. I would only emphasize that it can 
in any case by no means be regarded as definitely settled in the 
way in which Jackson puts it. 
In concluding this discussion of the phylogeny of the Echini 
it may not be inappropriate to give the following graphic represen- 
tation of my views on this highly interesting problem. I would, 
rately from the regular Echiui, states that ,von allen Autoren wird 
sehen.” ("Die biologiselkb Deutung å. Umgestaltung d. Echiniden” 
p- 49). He has then overlooked my above quoted paper. I am not 
quite sure that I understand Tornquist's reasons for the separate 
Voer na of Galeropus and Pyrina; but I shall not enter thereupon 
& this occasion. In any case it is interesting that the result of the 
diphyletie origin of the Irregular Echini has been arrived at inde- 
pendently by both of us. 
