223 
plates is a highly specialized feature; accordingly the more primitive 
biseriate arrangement cannot have been derived therefrom, and there 
is, in fact, no evidence, palæontological or embryological for such 
a derivation. After the primordial single series of (interambulacral) 
plates necessarily first follows a biserial arrangement and then gradu- 
ally the pluseriate arrangement develops therefrom; this order of 
events is actually found in the ontogeny of the Perischoechinoids, 
as Jackson has shown beyond doubt. However, I do not think 
it proved definitely herewith that the biserial arrangement is a 
more primitive type -than the pluseriate arrangement. I am here 
thinking only of the interambulacra; in the ambulacra, where a 
monoserial arrangement is -unknown and certainly never existed, the 
biserial type, of course, is the primitive, the pluseriate type the 
more specialized arrangement. I then quite agree with Jackson 
that the Perischoechinids with pluriseriate ambulacra, viz. the Palæ- 
chinidæ and the Lepidesthidæ, represent types which have left no- 
descendants beyond the Palæozoic times. With the Archæocidaridæ 
and Lepidocentridæ, which have both biserial:ambulacra, the matter 
lies quite differently. 
I would first. point out the peculiar fact that, while in those 
families with pluriseriate ambulacra the primitive forms with bi- 
seriate ambulacra, from which the development has necessarily 
started, are well known (Palæechinus, Lepidechinus), there is not 
known a single palæozoic Echinoid with two series of interambu- 
lacral plates (excepting Miocidaris cannoni Jackson and keyserlingi 
(Geinitz) which cannot come into consideration here, being from the 
Lower Carboniferous and Permian). According to Jackson's view 
such forms must have existed before the "forms with pluseriate 
interambulacra and have given rise to the latter. How remarkable 
that no trace at all appears to have been left of these forms! The 
reason of this is, in my opinion, not to be sought for in the im- 
perfectness of the geological record but in the non-existence of 
such forms. It is true that after the primordial interambulacral 
plate in the Perischoechinoids follow first two, then three plates 
