49 
may be practical, but it should be remembered that it is not a 
very essential distinction. — I shall not here enter otherwise on 
the question of the ancestry of the Echinina. 
Turning now to the ,,Irregularia", we must first make it 
clear, where we shall have to seek for the central or suranal plate. 
Lovén sought it in the apical system (the madreporite), while 
Agassiz (Calamocrinus p. 82—83) sought it among the plates 
of the periproct. Both of these eminent authors are evidently 
so far right, as the Irregularia may be supposed to have been derived 
either from such forms as the Salenids, in which the suranal plate 
is no longer part of the periproct, or from such forms in which the 
suranal plate is not at all developed, or in any case remains 
within the periproct. We may then seek for åa central or suranal 
plate in the Irregularia either among the plates of the apical system 
or among those of the periproct. | 
The Holectypoidea. Lovén (Études, p. 81) considers these 
forms as having a distinct central plate;” but the figures he gives 
of the apical system of Holectypoids show that it is only the madreporic 
plate (genital 2) which is sometimes conside- 
rably widened, as in Pygaster dilatatus (fig.. 9, 
p. 80), so as to occupy the middle space of 
the apical system. In others (Holectypus de- 
pressus Cott., Discoidea conica Desor, Pl. XV. 
Figs. 132—133) this plate is scarcely larger 
than the other genitals (Fig. 10). It was probably Fig. 10. Apical sy- 
stem of Discoidea 
i j conica Des. (From 
the ethmolytic Amphisternata as a compound plate, Lovén). 
the explanation of the large madreporie plate of 
homologous to the genital 2 +- the central plate 
- the genital 5, which induced Lovén to regard also the madre- 
poric plate of the Holectypoidea as a compound plate, homologous 
ter the genital 2 + the central plate. That this explanation of 
the madreporic plate in the Amphisternata is wrong, will be shown 
below; that it is no more natural for the Holectypoidea, seems 
evident from an unprejudiced inspection of the figure of Discoidea 
Vidensk. Meddel. fra den naturh. Foren. 1911. Bd. 63. 
JU: Bot, Garc 
1912 
