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impossible that such a form should not have had a long series of 
ancestors, whether it be derived from the Lepidocentrids, as I 
think, or from the Cidarids (directly or indirectly), as would 
probably be maintained by Jackson. But in any case no con- 
necting forms are known. The geological record is imperfect, also 
as regards such forms as the Echinoids, and the fact that con- 
necting forms are not known from the named periods can by no 
means outweigh the morphological arguments for the genetic relation 
between the two groups. 
In spite of the reasons Jackson has produced against the 
relationship between Echinothurids and Lepidocentrids I thus feel 
more convinced than ever that the Lepidocentrids are the direct 
ancestors of the Echinothurids. Whether it is then preferable to 
unite these two families into one group, the Streptosomata, or to 
unite the Echinothurids with the Diadematoids into one group, the 
Aulodonta, is a matter of minor importance; it depends on which 
features are regarded as the most important. I, for my part, think 
that the character of the ambulacral plates covering the peristome 
is more important than the differences in the interambulacra and 
the masticatory apparatus, and therefore prefer to unite the Lepido- 
centrids and the Echinothurids into one group, the Streptosomata, 
this classification having also the advantage of emphasizing the 
genetic relation between the palæozoice and the recent forms. On 
the other hand I am equally convinced that the Echinothurids are 
closely related to the Diadematids and probably gave rise to the 
latter, being thus in full accordance with Dåderlein (Op. cit.) 
and A. Agassiz & H. L. Clark (The Echinothuridæ p. 141) who 
maintain the close relationship between these two groups as the 
result of their investigations. 
Jackson's view that the Diadematoids have deveioped from 
the Cidaroids, however, gets important support from another side, 
viz. from Bather and Tornquist. Bather (Triassic Echino- 
derms of Bakony; p. 254) arrives at the conclusion that the Diade- 
moida "and the remaining Ectobranchiata” were derived from the 
