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THE VOYAGE OF H.M.S. CHALLENGEE. 
teeth. The disk is pierced with several — up to twelve — smaller peripheral holes 
around the large central ones, and has never, or at least very seldom, the margin 
smooth, but angular, or provided with small processes. A single Polian vesicle and 
madreporic canal. The genital organs attached far posteriorly, about 20 mm. 
behind the tentacles. Calcareous ring small, slender, about 3 mm, high, with 
the posterior margin wrinkled or angular, but devoid of posterior prolongations. 
The ten pieces of the ring are simple and slightly excavated posteriorly. 
Thyonidium marionii {Cucumaria), von Marenzeller, 1878. 
Habitat — Mediterranean Sea (von Marenzeller, Marion, Ludwig). 
In a letter von Marenzeller kindly informs me that his Cucumaria marionii is a 
Thyonidivm with twenty tentacles, the smaller pairs having at first escaped his 
attention. Considering the smallness of the only individual hitherto obtained, 
it is very probably a young one, which also explains the absence of pedicels on 
the interambulacra ; the species must he allied to the former, if not identical with 
it. Its deposits consist of tables with a rounded rhomboidal regularly perforated 
disk and a spire built up of only two rods. The radial pieces of the calcareous 
ring not so deeply excavated posteriorly as the interradial. 
2. Calcareous ring of ten simple or compound pieces, always with five radial 
posterior bifurcate prolongations, made up of several separate parts or 
joints. 
Thyonidium cebuense. Semper, 1868. 
The posterior prolongations of the five radial pieces of the calcareous ring are composed 
of three separate pieces or joints. The five interradial pieces terminate anteriorly 
in a single conical ''point, the radial in four small ones. 
Habitat . — Cebu (Semper). 
The tables, not described but only figured by Semper, seem to consist of a roundish 
disk perforated with one central and several, about eight, peripheral holes and 
a spire of four rods. 
Thyonidium magnum, Ludwig, 1882. 
Calcareous ring “ elongated posteriorly and spirally coiled.” The tables consist of 
a disk perforated with four central and numerous much smaller peripheral 
holes ; their spire is built up of two rods terminating in two to four points and 
strengthened by one transverse beam. Anteriorly, the ambulacra alone carry 
pedicels. 
Habitat . — Amboina (Ludwig). 
From the summary description of the calcareous ring, one cannot get an exact idea of 
its shape, whether it is composed of a large number of small pieces. 
