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 fact. I have shown”) that in Abatus 
cavernosus the genital 5 really exists in the young stages, and 
perhaps it may not be too hardy to conelude that it will also prove 
to exist in other Spatangoids. On the other hand I would. point 
out that the fact, that in some Pourtalesiids the posterior ocular 
plates are as rudimentary as the posterior genital plate of Spatan- 
goids, might with equal right be regarded 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 fact 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 apply Jackson's interpretation 
of the interambulacra to the single series of interambulacral plates 
in Bothriocidaris? Can the single series of plates belong to both 
the neighbouring ambulacra? It seems evident enough that here 
the interambulacra have no such relation to the oculars even if 
they be in contact with two oculars, and since we cannot, of course, 
suppose, that the interambulacra 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 interambulacral 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. Pl. 68. fig. 5 (of Lepidesthes formosa Miller). 
7") The Echinoidea of the Swedish South Polar-Expedition p. 76—77. 
