55 
ÅJ 
which are in accordance with this derivation. The tubercles of 
the Cassiduloidea are imperforate, as in the Salenids, while in the 
other ,,Irregularia" they are perforate. The other fact relates to 
the very important discovery recorded by A. Agassiz") that the 
young Echinoneus has teeth and a lantern, which are again resorbed 
in the course of the development. As seen by the figures of the 
paper quoted (Pl. II, figs. 6 and 7) the teeth are keeled, as in 
the Salenids, while in the Diadematids the teeth are unkeeled. 
Perhaps still more facts speak for this derivation of the Cassi- 
duloidea; but as I have not yet studied this group of Echinoids 
more closely, I will not at present enter on a more detailed dis- 
cussion of this question. 
Agassiz has suggested (Calamocrinus, p. 82) that the anal 
system of Æchinoneus may originally have been covered by a 
single large suranal plate, as he has found it in Echinarachnius. 
This is highly improbable, for several reasons. The anal area of 
the grown specimens do not show the slightest trace of such a 
plate. Further it is scarcely possible that both a central plate in 
the apical system and a suranal plate in the anal area could be 
present in any Echinoid. Now, it is true, a central plate is not found 
in the apical system in any of the recent Cassiduloids; but in 
view of the fact that this plate occurs in the older forms the 
suggestion lies near that it will prove to exist in the young of 
the recent forms. In any case the study of the development of 
the apical and anal system of the Cassiduloids will be of unusually 
great interest. 
Having thus revised the question of the existence of a suranal 
or central plate in the different groups of Echinoids, we may sum- 
marize the results as follows: 
A central or suranal plate has not been found in the Palæ- 
echinoidea, the Cidaroidea, Diadematoidea, Echinothuridæ, Arbaciidæ, 
Hojechspelken: the Meridosternata or Amphisternata, while its existence 
1) OR le rr of teeth and of a lantern in the genus Echinoneus 
Van Phels. Amer. Journ. Sci. XXVIII, 1909, p. 490. 
