222 
classification represents a decided step away from the recent towards 
the old view. His order Perioschoechinoida, which comprises all 
the pluriseriate forms (excepting the Echinocystoida, the Plesio- 
cidaroida and the Cretaceous Tetracidaris ) is placed apart from all 
the biseriate forms and represented as having no nearer relation 
to any recent group. Bothriocidaris is regarded as the most pri¬ 
mitive form, which has given rise to four separate oiders: the 
Echinocystoida, the Perischoechinoida, the Plesiocidaioida and the 
Cidaroida; from the latter the order Diadematoida (“Centrechinoida”) 
is derived, and from one of the suborders of this order, the Stiio- 
donta, is derived the order Exocyclica. 
Against the primitive character of Bothriocidaris I think there 
can be raised no objection. Not only is it the oldest Echinoid 
known, but it also decidedly represents the simplest type of Echi¬ 
noid structure known; especially the single column of interambu- 
lacral plates must be regarded as a very primitive feature, recapi- 
tulated in the ontogeny of all other Echini, the interambulacra always 
beginuing with a single primordial plate the Piotechinus 
stage, as designated by Jack s on 1 ). That Bothriocidaris represents 
the ancestral type, from which all the rest of the Echini have 
developed, seems then beyond doubt. Likewise it can scarcely be 
questionable that the Echinocystoida — judging from the imperfeet 
knowledge we possess of them and on the premiss that their 
exocyclic character is a reality 2 ), — represent a special bianch, 
which has not given rise to the development of other types. But 
here my accordance with Jack s on in the views of Echinoid phy- 
logeny stops. 
Jackson’s main argument for regarding the Perioschoechi¬ 
noida as a group which has no descendants among the rest of the 
Echinoids is this, that the pluriseriate arrangement of the coronal 
1) E. T. Jack son. Studies of Palæechinoidea. Bulk Geol. Soc. America. 
Vol. 7. 1896. p. 285. In the “Phylogeny” he only uses this name 
occasionally (p. 210). 
2 ) I am not very mueh inelined to think it is correct. 
