56 
in the Clypeastroidea is still uncertain (Echinarachnius !). Only in 
the Acrosalenidæ, the Salenidæ, the Echinina and the Cassiduloidea 
is it typically present. 
It seems very probable that the central (suranal) 
plate has developed separately along different lines; 
thus the suranal plate of the Echinina can scarcely be derived from 
the central plate of the Salenids, while on the other hand the central 
plate of the Cassiduloids may probably be derived from that of 
the Salenids, thus indicating a different origin of this group from 
that of the other Irregularia, which evidently have been derived 
from the Diadematoidea through the Holectypoidea. 
Thereis no evidence to support the theory that the 
anal area of the Echinoids was originally covered by 
five radial anal plates (,,infrabasalia”). The primary condition 
is, according to all evidence, an indefinite number of plates arranged 
more or less in circles, but without any relation to the radii or 
interradii. The suranal plate of the Echinina, the central plate of 
Salenids, the four anal plates of Arbacia etc. probably all originated 
through special growth of some of the anal plates, these plates 
being thus all of the same morphological value. 
The central plate can by no means beregarded as 
forming an essential part of Echinoid morphology, 
being not a primitive, but aspecial feature, acquired 
separately in some of the more specialised groups, 
while it is not found in any of the more primitive 
groups. Accordingly ,,we are no longer justified in looking upon 
this kind of anal system with a single plate as the earliest type" 
(Agassiz. Calamocrinus, p. 79). — I may quote the following pas- 
sage from Neumayr!): ,,S0 ergibt sich, dass alle geologisch alten 
Formen in der entschiedensten Weise gegen die Auffassung sprechen, 
dass eine Apexentwicklung, wie sie bei Sa/enia persistirt, als typisch 
fir die ganze Abtheilung der Seeigel betrachtet werden kånne.... 
1) Morphologische Studien iber fossilen Echinodermen. Sitz. Ber. Kais. 
Akad. d. Wiss. Wien. Bd. 84. I. 1882, p. 152. 
