CLASSIFICATION OF PALEOZOIC ECHINI. 243 



The 4 columns of ambulacral plates of Echinocystites would on our 

 accepted basis of classification associate this type with Oligoporus as 

 the only other type known which has such a structure ; but the fact 

 that the genus occurs in the Upper Silurian is very strong evidence 

 against such association. The interambulacra have few columns of 

 plates at the ventral area, as shown in the published figures, and pro- 

 ceeding dorsally new columns are introduced until the full number is 

 attained. The imbrication of the plates and eccentric anal area, together 

 with the small peristome which is gathered from Sir Wyville Thomson's 

 figures and description, would indicate that this type is aberrant, for we 

 know these features, particularly when in association, only in aberrant 

 forms. On the whole evidence it seems much the safer course to leave 

 this order by itself rather than place it near forms which have appar- 

 ently similar ambulacral detail. To place it near Oligoponis would on 

 the evidence be a blind following of system. 



Order IV, Plesiocidaroida. — This order, which is the last group to be con- 

 sidered, is represented only by the one Triassic genus, Tiarechinus. This 

 type has been fully illustrated by Professor Loven (26). In its structural 

 detail this form differs from all the Perischoechinoida and therefore 

 naturally forms a separate order by itself. The most important char- 

 acter of this type is the fact that, while it has a single interambulacral 

 plate in the first row of the corona like other echinoids, in the second 

 row it has 3 plates. All other types above Bothriocidari!^, as far as known, 

 have 2 plates in the second row, and the existence of 3 in Tiarechinus is 

 therefore to be looked at as a feature standing quite by itself as a structural 

 detail. After the second row, no more interambulacral plates are added 

 in Tiarechinus. 



The accompanying table includes all genera of Paleozoic Echini except 

 Xenocidaris, which is known only from spines (page 222), Spatangopsis,^ 

 KonincJcocidaris,^ and Echino discus, \ which are imperfectly known. Of 

 the genera tabulated all the species are included excepting species which 

 have been described from dissociated spines and plates or fragmentary 

 .specimens, which with present knowledge are wanting in the characters 

 for proper systematic interpolation. The known species of the included 

 genera which are omitted in the table are Lepidocentrus eifelianus and 

 quite a large number of species of Archseocidaris. In the table the num- 

 bjers of the orders and the spelling of class and ordinal names are adopted 

 from Dr Duncan's " Revision of the Genera and Great Groups of the 

 Echinoidea." 



♦Which have not been figured and are not sufficiently understood to be included, 

 t Worthen and Miller (42). Two species are ascribed to this genus. 



