29 
such an advance would not be accompanied by the dwindling and 
disappearance of the sur-anal. Because the clavicles are small or 
entirely absent in the mostly extinct Ratitæ, while in all cases 
well-developed in the mainly recent Carinatæ we can not pro- 
nounce them unessential features of vertebrate morphology“. 
This reasoning might, perhaps, be acceptable in case the 
Palæechinoids were really the only Echinoids in which the central 
plate is lacking — though several severe objections might be 
raised against it; this being, however, not the case, the whole 
argumentation falis short, and the only natural explanation of the 
facts is to assume that the central or suranal plate has not yet 
made its appearance in the Palæechinoids. 
The plan of the present note is not to give a historical 
account of the question, only to give a critical examination of the 
central (suranal) plate in the different groups of Echinoids, in 
order to show whether it is really an essential feature of Echinoid 
morphology, as maintained by Clark (in accordance with Lovén 
and P. H. Carpenter), or it is a late acquisition in Echinoids, 
developed independently liere, not inherited from the ancestors of 
Echinoidea, as I think (in accordance with A. Ag as s i z x ), the Sara- 
sins, Neumayr, Jaekel, Semon, Gr ego ry, MacBride, 
*) In the «Challenger» Echinoidea (p. 18) Agassiz says: „As Loven 
has well shown in the older Echinids (Cidaris and Salenia), we 
find all the proof we need of the crinoidial character of the apical 
system of the Echinidæ; the calyx being more and more unimportant, 
though it always reveals its typical features 14 . Later on he has 
chauged his opinion on this matter. In his Monograph on Cala- 
mocrinus he declares that the apical system of Echinoids with a 
single large suranal plate does not represent the primitive condition 
and that this plate cannot be homologized with the central plate of 
Crinoids. „In the Crinoids the central area is occupied by a stem or its 
representative, and it seems to me more natural to homologize this 
central area of the Echini and of the Crinoids than to attempt, as has 
been done, to pick out a single anal plate of the Echini which does 
not exist in many recent families, and probably not in many fossil types, 
and homologize it with the central plate of Starfishes or of Ophiurans 
and the terminal plato of the stem of the larval Comatulæ, as has 
been done by Carpenter 44 (Op. cit. p. 69). 
