236 
pyramids, keeled teeth, auricles and sphæridia. He might have 
added, that also the pedicellariæ then afford several cases of parallel 
development. Of these structures I do not see any difficulty in 
assuming the parallel . development of the biseriate interambulacra 
and the compound ambulacral plates; indeed, I have in my previous 
works on Echinoids repeatedly emphasized that the compound am- 
bulacral plates must have arisen independently in different groups. 
Likewise I see no difficulty in assuming that the ambulacral plates 
of the peristome have been reduced to a single pair in different 
groups. That the dental apparatus has developed in a similar way 
in the different groups does not appear to me so very improbable 
either; indeed, I think there is proof that keeled teeth have devel- 
oped independently in different groups. As shown by Haw kins”) 
the teeth of Discoidea cylindrica were keeled, but Discoidea is un- 
doubtedly derived from the Diadematoids with unkeeled teeth, in 
any case on a different line from the other Echinoids with keeled 
teeth. That the peristomial gills developed together with the dental 
apparatus, is very naturally explained from their physiological con- 
nection with this apparatus (cf. above p. 226, note). More difficult to 
explain is perhaps the parallel development of sphæridiæ and pedi- 
cellariæ. It would certainly be more satisfactory not to have to 
account for these features as parallelisms. But the structure of 
the spines seems to me to be best explained on the assumption of 
the genetic connection between the Stirodonta and the Cidaroids. 
Before coming to the last part of Jackson's work I must 
say some words on the nomenclature. In general I think the names 
chosen are those which ought to be adopted. I am especially 
glad to see that Jackson retains the excellent name Årchæo- 
cidaris, not adopting that quite misleading name Echinocrinus, 
which has, strictly taken, (the priority. But while here Jackson 
choses the only reasonable way, he unfortunately has acted in 
quite another way in regard to the still more important name 
1) H. L. Hawkins. On the jaw apparatus of Discoidea cylindrica (La- 
marck). Geol. Magaz. Now. Ser. Dec. 5 Vol 6. 1909. p. 148 
