233 
development; in the rest of the Stirodonta this outer layer has 
quite disappeared. 
The Exocyclica Jacks on derives as a whole from the Stiro¬ 
donta, more especially the Arbaciids, however without going more 
into detail m regard to this interesting part of the phylogeny of the 
Echini. Against this derivation some objections may be raised. It is 
undemable that in some diadematoid forms ( Pygaster ) we may trace 
the transition from the Diadematoid type to the primitive exocyclic 
type, the Holectypoidea: in faet it is difficult or almost impossible 
to draw a distinet limit between the Holectypoidea and the Diade- 
matoidea. This faet is decidedly in favour of the suggestion that 
we have liere really the ancestors of at least a part of the Exocyc¬ 
lica. Further it is an important faet that in the Holectypoids, as 
also in the Spatangoids the tubercles are perforate 1 * ), which is 
doubtless a primitive feature. But in the Stirodonta the tubercles 
are imperforate. In deriving the Exocyclica from the Stirodonta 
we would then have to assume the primitive perforate tubercles to 
have developed from the more specialized imperforate tubercles of 
the latter, which is certainly not very probable. 
However, I think it quite probable that also the Stirodonta 
have given rise to some Exocyclica. As I have set forth in my 
“Echinological Notes” HL*) there are reasons for the suggestion 
that the Cassiduloids have developed from the Salenids. The 
tubercles are imperforate, the teeth are keeled and there is a 
di stinet suranal plate; these important facts are certainly in favour 
of the Salenid origin of this group. The Exocyclica thus probably 
had a diphyletic origin 3 ). 
a ) Jackson states (p. 78) that the primary tubercles are imperforate 
in the Holectypina and “many of the Spatangina”. This is incorrect. 
Among the Exocyclica imperforate primary tubercles occur only in 
the Cassiduloidea. 
-) The central (sur-anal) plate of the Echinoidea. Vid. Medd. Naturh. 
Foren. København 1911. 
3 ) Tornquist, who likewise has come to the conclusion that the Cassi- 
dulids (he names only Galeropygus and Pyrina) have arisen sepa- 
