48 
als Salenigonen, indem wir sie fir Descendenten der Saleniden 
halten". (Uber die Anat. d. Echinothuriden u. die Phylogenie d. 
Echinodermen, p. 150). This does not appear quite so' convincing 
as the Sarasins seem to think. I would especially emphasize 
the difference, already pointed out by Agassiz (cf. p. 46), that 
in the Salenids the suranal plate is outside the periproct, while 
in the Echinina it remains within the periproct. This fact un- 
doubtedly leads to the conclusion that the suranal plate of the 
Salenids represents the more specialized condition; but the more 
primitive condition cannot be derived from the more specialized. 
The same may apply to the periproct itself. In the Echinina it 
. is central, though sometimes with a slight 
tendency towards becoming excentric in 
the direction of the ocular I and genital 
I, as indicated by the fact that the 
ocular I is generally the first: to come 
into contact with the periproct. Also 
the anal opening has a— still more — 
distinct' tendency towards becoming ex- 
Fig. 9. Apical system of centric in the same direction. This is 
Gymnechinus pulchellus. 
(From Th. Mortensen. carried to an extreme, both as regards 
Siam-Echinoidea, I. Pl. II. the periproct and the anal opening, in 
Svag the genus Gymnechinus (Fig. 9). — In 
the Salenids the periproct is excentric; in the recent forms the 
excentricity is in the same direction as in those of the Echinina, 
where an excentricity can be observed at all; but in the older 
forms it is excentric in the direction of the posterior inter- 
ambulacrum (genital 5). This also is against the derivation of the 
Echinina from the Salenids. It therefore seems most probable 
that the suranal plate of the Echinina is a feature acquired sepa- 
rately by this group of Echinoids. There may thus far be reasdn 
to distinguish between this plate in the two groups, and I would 
suggest that the plate of the Salenids be termed the central 
plate, that of the Echinina the suranal plate. This distinction 
