49 
may be practical, but it should be remembered that it is not a 
very essential distinction. — I sliall not liere enter otherwise on 
the question of tke ancestry of the Eckinina. 
Turning novv to tke „Irregularia 14 , we must first make it 
clear, where we skall have to seek for the central or suranal plate. 
Lov én sought it in tke apical system (the madreporite), wkile 
Agas si z (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 a central or suranal 
plate in the Irregularia either among the plates of the apical system 
or among those of the periproct. 
The Holectypoidea. Lovén (Etudes, p. 81) considers these 
forms as having a distinet 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 dilataius (fig. 9, 
p. 80), so as to occupy the middle space of 
the apical system. In others ( Holectypus cle- 
pressus Cott., Discoidea conica Desor, PI. XV. 
Figs. 132 —133) this plate is scarcely larger 
than the otker genitals (Fig. 10). It was probably 
the explanation of the large madreporic plate of 
the ethmolytic Ampkisternata as a compound plate, 
homologous to the genital 2 -4- the central plate 
4" the genital 5, which induced Lovén to regard also the madre¬ 
poric plate of the Holectypoidea as a compound plate, homologous 
to the genital 2 -j- 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. 4 
Fig. 10. Apical sy¬ 
stem of Discoidea 
conica Des. (From 
Lovén). 
