240 
is certainly very improbable. Herewith I think the mytb of Al- 
drovandus having already known the excessively rare Bothrio- 
cidaris may be safely dropped. 
Apart from this I have nothing specially to critizise in this 
part of the work. Of considerable interest is the explanation of 
the plates regarded by Sollas and Spencer as roofing over the 
ambulacral plates in Palæo dis cus ; Jack son shows conclusively 
that they are only the narrower ambulacral plates of the dorsal 
side “brought in mechanical contact with the wider ventral plates 
by the flattening of the test” (p. 251). The connection between 
Asteroids and Echinoids- based upon these “roofing” plates is thus 
shown to be without real foundation. (I am likewise satisfied to 
see that Jack s on rejects the hypothesis of*A. H. Clark of a 
nearer relation between Echinoids and Crinoids). For the rest it 
would carry much too far to mention more closely the many im- 
portant facts found in this part of the work; I would simply name 
such points as the comparative study of the ambulacra in the Pa- 
læechinidæ (diagrammatically represented on p. 231), the description 
of the masticatory apparatus in se veral palæozoic forms ( Archæo - 
cidaris rossica, Pholidechinus brauni, Meekechinus elegans, the latter 
presenting the unique feature of having serrate teeth), and tlie 
numerous test-analyses after the mode of Lovén (e. g. Archæo- 
cidaris rossica , Hyattechinus beecheri , Palæechinus quadriseriahs, 
Melonechinus multiporus etc.). 
The conclusion must be: most sincere congratulations to Pro¬ 
fessor Jack son for the completion of this monumental work. 
