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THE VOYAGE OF H.M.S. CHALLENGER. 
(?) Thyone elongata [Thyonidium), Ayres, 1851. Orcula elongata. Semper, 1868. 
Habitat. — George’s Bank (Ayres). 
Thyone jioccosa, Norman, 1864. 
Habitat. — British Islands (Norman). 
Genus 10. Stereoderma, Ayres, 1854. 
Tentacles ten, two ventral smaller. Ambulacral appendages in the shape of pedicels, 
scattered irregularly over the body, but having a tendency to form a regular 
double-row in some parts of the trivium. Anus without teeth. Integument 
hard. 
Stereoderma unisemita [Anaperus), Stimpson, 1854; Ayres, 1854; Selenka, 1867. 
Cucumaria fusiformis, Desor, 1851 (according to Verrill). 
Calcareous ring, like that in Psolus phantapus, of ten simple pieces without posterior 
prolongations. Deposits — numerous crowded, thick, oval, smooth plates or rather 
buttons with four holes. Pedicels in want of terminal plate and nearly destitute 
of other deposits. The double row of pedicels is somewhat irregular, and placed 
on one side (according to Stimpson), or forms a well-defined line beneath (Ayres), 
or forms a double row along the right (or left) side of the body (Selenka). 
Habitat. — Grand Bank and the coast of New England (Stimpson), Newfoundland and 
Massachusetts (Ayres), off Martha’s Vineyard (Verrill), Nantucket (Desor), 
(?) Cape Palmas (Selenka). 
Stereoderma murrayi, Bell, 1883. 
The double row of pedicels only well developed in the anterior third of the body. 
Calcareous ring composed of numerous small plates, and carrying five long 
slender prolongations posteriorly. Deposits — numerous crowded, oval plates, 
with uneven or undulated margin, pierced with four holes and provided with 
lower or higher elevations round the margin ; on the upper and under surfaces 
a half-ring is placed so as to form together a complete smaller ring, vertical in 
position to the plate itself. 
Habitat. — Kurrachee (Bell). 
I am almost convinced of the identity of the two forms Stereoderma murrayi and 
Thyone sacellus, which supposition is supported by the presence of the thick hard 
perisome with its unusually shaped deposits, the characteristic calcareous ring, 
&c. Some very unimportant differences exist : — Stereoderma murrayi is of a 
