232 
the primordial interambulacral plate retained as in the Lepidocen- 
trids, only more or less reduced in larger specimens. Also in the 
Diadematids the primordial interambulacral plate is certainly present 
in young specimens — as it is upon the whole in all young 
Echinoids — the only difference being that in the Diadematoids 
it is resorbed at an early point of the development, in the Echi- 
nothurids only partially, while in the Lepidocentrids it was retained 
in its original shape during life. There is here a gradation agreeing, 
indeed, most beautifullv with what must be required on the assump- 
tion of a direct genetic connection between these three families, 
and there is not required a series of unknown forms to bridge 
over the gap; in faet, there can scarcely be said to be a gap in 
the series Lepidocentridæ—Echinothuridæ—Diadematoidea. As for 
Bather’s other argument, that it is impossible to decide whether 
such forms as the Triassic Mesodiadema and Diademopsis are 
really Cidarids or Diadematoids, because they are intermediate 
between these two groups, I would yenture to think that the 
difficulty is due to the imperfeet knowledge of these forms; as 
Bather himself says: “were the complete test, and still more the 
li ving animal, preserved to us, the difficulty might not occur” (Op. 
cit. p. 254). To base so “violent” a hypothesis on some few 
fragments of tests and some doubtful radioles against all other 
evidences afforded by both recent and, mueh better known, fossil 
forms, I really find unjustifiable. 
While thus I cannot agree with Jack s on, Tornquist and 
Bather in assuming that the Diadematoids were derived from the 
Cidaroids, I am inelined to think on the other hånd, that the 
Saleniids, and then probably all the Stirodonta, are derived from the 
Cidaroids, not from the Diadematids. A main argument for this is 
the structure of the spines in the Saleniids, viz. the presence of 
an outer layer, ostraeum, corresponding to what occurs in Cidarids. 
It does not appear very probable that this structure has developed 
from Diadematid spines. The Arbaciids present traces of the 
same outer layer and thus appear to represent a further stage of 
