230 
Cidaroida, “however improbable it may at first have appeared”, 
founding on the faet that the Triassic Diademoids (Mesodiadema , 
Diademopsis) represent transitional forms between these two groups, 
it being, in faet, almost impossible to decide (from the material 
available) whether they are really Cidarids or Diadematids. The 
“less violent hypothesis” that the Ectobranchiata might derive from 
the Lepidocentridæ, in which, as in the Diademoids, the interam- 
bulacra are absent from the peristome, both differing in this im- 
portant point from the Cidarids, is rejected on the following ground. 
In the Lepidocentridæ the primordial interambulacral plate is retained 
in the adult, “whereas the Diademoida are clearly derived from 
forms in which the adoral interambulacrals have disappeared”, the 
absence of interambulacral plates from the peristome in these two 
groups being thus apparently due to different causes. “A consider- 
able series of genera would therefore be required to bridge the 
gap between the Lepidocentridæ and the Diademoida, but of this 
postulated series no representatives are known. This hypothesis 
therefore would present more rather than fewer difficulties.” 
Tornquist 1 ), independently of Bather, has arrived at the 
same conclusion, that the Diadematoids have been derived from the 
Cidarids. He founds this conclusion on the faet that the primitive 
Diadematoids (“Prodiadematidæ”) have simple ambulacral plates like 
those of the Cidarids, the most primitive type being that with the 
porepairs horizontally arranged, the next stage having them obliquely 
placed (“oligopore”, as he erroneously designates it; see above p. 218). 
In the last paper quoted he emphasizes his agreement with Bather 
in regard to the phylogeny of these groups, stating that „ein 
Zweifel dariiber, dass die gesamte Gruppe der Diadematoidea und 
daruit auch indirect die Irregulares von den Cidariden abzuleiten 
ist, kann auch nach dem Urteil Bather’s nicht mehr bestehen” 
i) A. Tornquist. Die Diadematoiden des wurttembergischen Lias. 
Zeitschr. d. deutschen geol. Ges. 60. 1908. p. 378. Die biologische 
Bedeutung der Umgestaltung der Echiniden im Palæozoikum und 
Mesozoikum. Zeitschr. f. indukt. Abstamm.- u. Yererbungslehre VI. 1911. 
