225 
ø 
been carefully studied as regards the first formation of the coronal 
plates. It is thus by no means impossible that traces of a pluri- 
seriate condition may yet be found in the development stages of 
recent Echini; especially it would be interesting to investigate the 
Echinothurids in regard to this question. Perhaps also Pygmæo- 
cidaris might be regarded as a case of atavism, recapitulating the 
ancestral pluriseriate condition. — But even if no embryological 
evidence for the primitive character of the pluriseriate condition 
can be found, I think the reasons given above for that opinion 
outweigh completely Jackson's reason for regarding the pluri- 
seriate condition as the most specialized, viz. that a pair of bi- 
seriate plates are actually seen below the pluriseriate. There is no 
proof that this really represents a phylogenetic stage; as stated 
above it is sufficiently explained by the conditions of space. 
Accordingly the pluriseriate interambulacra of the Perischo- 
echinoids, in my opinion, represent a primitive condition, not a 
very specialized, and there is herein no reason, why the Archæo- 
cidaridæ should not have given rise to the Cidaridæ and the Lepi- 
docentridæ to the Echinothuridæ and the Diadematoidea. The Ar- 
Chæocidaridæ so closely resemble the Cidarids in all their characters 
that, in fact, the pluriseriate interambulacra make the only essential 
difference. The only natural thing is then to regard the Archæo- 
Cidarids as the direct ancestors of the Cidarids. Jackson's dis- 
Covery of a true Cidarid, Miocidaris cannoni, from the Lower 
Carboniferous, is, of course, in no way in contradiction with this 
view. I am glad to agree herein completely with Bather, who 
states (Triassic Echinoderms of Bakony, p. 251) that "there is no 
reason to doubt that Archæocidaris is a true ancestor of Miocidaris.” 
Tornquist (Op. cit. p. 32) likewise states ,,dass sie (die Archæo- 
cidariden) mit Sicherheit als ihre (der Cidariden) direkten Vor- 
låufer angesehen werden kånnen.” 
In the ''Siam-Echinoidea” I (p. 53) I suggested that the 
Echinothurids have developed from the Lepidocentridæ, in which 
the ambulacral plates cover the peristome in the same way as in 
Vidensk. Meddel. fra Dansk naturh. Foren, Bd. 65. 15 
