215 
portant a conclusion on cases of aberrant variation, which are, in 
spite of the ingenious way in which Jackson treats them, only 
abnormalities. Further the absence of genital 5 in Spatangoids 
is perhaps not quite a sure faet. I have shown *) that in Abatus 
eavemosus the genital 5 really exists in the young stages, and 
perhaps lt may not be too hardy to conclude that it will also prove 
to exist in other Spatangoids. On the other hånd I would point 
out that the faet, that in sorne Pourtalesiids the posterior ocular 
plates are as rudimentary as the posterior genital plate of Spatan¬ 
goids, might with equal right be regardod as leading to the inverse 
conclusion: that the ocular plates have nothing to do with the 
development of the ambulacral plates. There is, however, another 
and more important faet which is decidedly not in favour of Jack- 
son’s interpretation. 
It is emphasized by Prof. Jackson as one of the main results 
of his researches that Bothriocidaris is the most primitive of all 
known Echinoids, from which all the rest must be derived and I 
quite agree with this. But how to applj Jackson’s interpretation 
of the mterambulacra to the single series of interambulacral plates 
m Bothriocidaris ? Can the single series of plates belong to both 
the neighbouring ambulacra? It seems evident enough that here 
the mterambulacra have no such relation to the oculars even if 
they be in contact with two oculars, and since we cannot, of course, 
suppose, tbat the mterambulacra of Bothriocidaris have another 
value than those of other Echini, the interpretation of Jackson 
seems to me to lack real foundation. It is curious, indeed, that 
Professor Jackson does not appear to have thought of this point, 
obvious though it be. A form like Meekechinus, with three 
series of mterambulacra] plates, likewise affords difficulties for the 
interpretation of Jackson, as also some forms of the genus Lepi - 
desthes are not easily brought in accordance with his view; I 
might recall e. g. PI. 68. tig. 5 (of Lepidesthes formosa Muller). 
] ) The Echinoidea of the Swedish South Polar-Expedition p. 76-77. 
