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THE VOYAGE OF H.M.S. CHALLENGER. 
and differs from it in some characters of more or less importance. Generally 
it seems to be larger, up to 160 mm. long, with closely placed pedicels, and often 
with the ambulacra indicated by a longitudinal furrow. It almost appears to 
be a rule, that the tentacles deviate somewhat from what is the typical condition 
in this genus. The calcareous ring is rather high, up to 17 mm. in the largest 
specimens ; its radial pieces are excavated posteriorly so as to give rise to two 
short prolongations, but, considering that the ring in the specimens examined 
by me is much contracted and wrinkled, I am by no means sure of it, nor 
whether the pieces are simple or composed of some secondary parts, which some- 
times seems to be the case. Excepting terminal plates in the pedicels, the body 
should be devoid of deposits, but in some specimens I have found tables in the 
cervical portion of the body. A single madreporic canal and two or three 
Pohan vesicles. Norman says that the true Thyonidium commune, Forbes, differs 
from the Scandinavian form, described under the same name by Diiben and 
Keren, in having the skin covered with tables which have a nearly circular disk 
with numerous perforations and a spire built up of four rods. Now my own 
researches show that the Scandinavian form also has deposits in the shape of 
tables, though principally in the cervical portion of the body. Norman does 
not point out in what respects his Thyonidium commune differs from Thyonidium 
pellucidum, nor does he say whether he has examined the types of Forbes and 
Thompson. It appears very probable that his species is only a Thyonidium 
jpellucidum. 
Thyonidium productum [Duasmodactyla) , Ayres, 1854 and 1873 ; Stimpson, 1854. 
(?) Or cula punctata, Agassiz, 1852 (according to Stimpson). 
Habitat. — East Port (Ayres), Grand Manan (Stimpson). 
Deposits in the shape of “ irregular perforated plates ” present only in the cervical 
portion. Probably this species is not distinct from the preceding one. 
II. Deposits of the body-wall itself absent, or present in greater or smaller 
number in the form of spinous rods. 
Thyonidium mollc {P attains), Selenka, 1868; Semper, 1868. Thyonidium peruanum. 
Semper, 1868. Pattalus peruvianus, Verrill, 1867 to 1871. Anaperus 
peruanus, Verrill, 1867 to 1871. 
Tentacles varying from sixteen to twenty-one, sometimes typical; often, however, 
a typical in being of nearly equal size. Calcareous ring of ten simple pieces, 
the radial much higher than the interradial, and devoid, as it seems, of true 
posterior bifurcate prolongations. Deposits, when such are present, irregular, 
slightly spinous rods pierced by some holes. 
