FEBRUARY 16, 1912] 
atory evidence. But as there seems to be no 
really characteristic holothurian structure 
shown by the fossil, I fail to see why it 
should be considered a holothurian. 
The small specimen of Mackenzia is very 
suggestive of a synaptid without its tentacles, 
but the most searching examination fails to 
show a single character which gives positive 
support to this view. The mouth and its as- 
sociated structures are not distinctly indi- 
cated. As stated above, I could not distin- 
guish any separated or definite parts in the 
taised ring which seems to surround the 
mouth, and there is nothing in it to me sug- 
gestive of the caleareous ring of a synaptid. 
The longitudinal markings of the body-wall 
are more numerous and closer together than 
they should be if they indicate the longitudi- 
nal muscles of a holothurian. On the whole, 
these longitudinal markings, the appearance 
of the body surface and of the oral end, and 
the form of the animal all seem to me sug- 
gestive of certain actinians, although I do not 
assert that the fossil really represents that 
group. The larger specimen, referred to 
Mackenzia by Dr. Walcott, shows practically 
no structure and in my judgment can not be 
assigned positively to that or any other genus. 
Turning now to Hldonia, of which the ma- 
terial is plentiful and its condition such that 
the structure can be made out with a fair de- 
gree of completeness, we find an animal so 
medusoid in outer form that Dr. Walcott uses 
the terms “exumbrella,” “subumbrella,” “lo- 
bation ” and “lappets” and says “the system 
of radial canals is very striking and medusa- 
like.” I do not recall any medusa with a 
canal system like Hldonia’s, with a small cen- 
tral ring, but I think most of us will agree 
that the general appearance of the animal is 
that of a free-swimming Celenterate, except 
for the apparently distinct and extraordinary 
alimentary canal. It is upon the interpreta- 
tion given this structure and upon the import- 
ance attached to it, that our final decision as 
to the position of Hldonia must depend. Dr. 
Walcott at first thought it might be a com- 
mensal worm but later decided it was really 
the alimentary canal of the animal itself, and 
SCIENCE 
277 
upon the strength of its partially spiral form, 
he based his decision to call Hldonia a holo- 
thurian. He has, however, pointed out the 
essential differences between #ldonia and 
Pelagothuria, the only known free-swimming 
holothurian, making it plain that they are not 
at all nearly allied. Emphasis should be 
placed on the fact that except for the expan- 
sion of the oral disk as a swimming organ, 
Pelagothuria is not an extraordinary holo- 
thurian, its internal anatomy being like that 
of many other members of the class. Its ali- 
mentary canal is in loops (a long drawn out 
spiral) and the mouth is surrounded by the 
usual circle of tentacles. The alimentary 
eanal of Eldonia is not in loops as in a holo- 
thurian but seems to have been more nearly 
in a single plane like one half of the canal of 
a sea-urchin. The appearance of the tube 
thus seems to me more echinoid than holo- 
thurioid. The mouth of Hldonia has on either 
side a large tentacle; neither Dr. Walcott nor 
I have been able to find more than two and 
the whole appearance of the oral region 
indicates two as the normal number. The 
tentacles are described by Dr. Walcott as 
“veltato-digitate ” but they have almost noth- 
ing in common with the sort of tentacles to 
which that term has hitherto been applied.’ 
On the other hand they seem to me suggestive 
of the marginal clusters of tentacles in 
Imcernaria and its allies. Perhaps even the 
oral tentacles of some Rhizostomous Meduse 
are not fundamentally different. In some of 
the specimens, notably the one shown in Fig. 
1, Pl. 10, bits of the tentacles show some slight 
indications of their finer structure. In the 
figure referred to, small lobes or knobs above 
the central ring (CR) are noticeable and 
these, in the specimen, show, under the lens, 
a remarkable resemblance to clusters of 
nettle-cells. I am not sure that these lobes 
are part of the tentacles but if they are, as 
they seem to be, my opinion that the tentacles 
are more medusoid than like anything known 
among holothurians would be confirmed. 
Hildonia shows absolutely no trace of pen- 
™See Ludwig, 1892, ‘‘Die Seewalzen,’’ p. 97; 
Pl. VII., Fig. 5. 
