72 
make it a generic character seems to me unjustified. A. H. Clark 
has suggested in a letter to me that the rudimentary side and 
covering plates of the pinnules rnight afford a characteristic differ¬ 
ence, being comparatively well developed in petasus , while they 
appear to be lacking in the other species. This does, however, not 
hold good. It is true, they may be lacking in bifida, but this is 
no constant feature. J. Gr i eg 1 ) has given a figure showing them 
just as well developed in bifida as in petasus , and I find them 
quite similarly developed in some specimens of bifida , although 
they appear to be lacking in the majority of the specimens. Also 
in mediterranea I find them almost as well developed as in petasus. 
The necessary conclusion is then that there is no reason for a 
generic distinction of A. petasus from the other species of Antedon 
s. str., in spite of the remarkable differences in their embryology. 
We have simply to accept the faet that closely related forms may 
show surprisingly great differences in regard to their development. 
— A parallel case is that of the Echinoids Toxocidaris (or Helio- 
cidaris) tuberculata and erythrogramma , the former having a 
typical pelagic larva, the latter direct development without any in- 
dication of a pluteus-stage 2 ); another similar case is afforded by 
Amphiura filiformis and borealis , the former having small eggs 
and a typical Ophiopluteus larva while the latter is viviparous, 
having large, yolky eggs, the embryos doubtless developing direetly. 
In both these cases the species undoubtedly are closely related 
and must be referred to the same genus. 
I may take the opportunity of mentioning here that I have ob- 
served on specimens of A. petasus from the Swedish Zoological 
Station, Kristineberg, a small Loxosoma attached to the pinnules, 
generally between the tentacles, where it is by no means easy to 
see. I have not examined it more closely, so that I cannot say, 
Bjarnes A. G ri eg. Echinodermen von dem norwegischen Fischerei- 
dampfer „Michael Sars“ in den Jahren 1900—1903 gesammelt. II. Cri- 
noidea. Bergens Mus. Aarbog 1903. Nr. 5. p. 33. 
2 ) Th. Mortensen. Preliminary note on the remarkable shortened devel- 
opment of an Australian sea-urchin, Toxocidaris erythrogrammus. Proc. 
Linn. Soc. N. S. Wales. Vol. XL. 1912. p. 203. 
Th. Mortensen. On the development of some Japanese Echinoderms. 
Preliminary Notice. Annot. Zooiogicæ Japonenses. VIII. 1914. p. 545. 
