94 
THE VOYAGE OF H.M.S. OHALLENGEE. 
Colochirus as well as to Thyone, in consequence of the shape of its calcareous ring and 
calcareous deposits. 
Thyone recurvata, n. sp. (PL V. fig, 7 ; PL VIII. fig. 6). 
Body fusiform, tapered posteriorly into a long narrower caudal portion, curved, with 
the extremities turned upwards. Tentacles ten (?), retracted. Pedicels numerous, 
distributed all over the body, more densely crowded on the ventral surface, hard, stifi" 
and not retractile. Anus without teeth. Perisome hard, brittle from close-lying, large, 
irregular, smooth plates with numerous holes ; the plates overlap each other more or less. 
The exterior layer of the perisome contains a quantity of small cups, made up of an 
X -shaped body with the arms curved and their ends united to a spinous rim. Pedicels 
strengthened by about the same kinds of deposits, though the plates are more deformed 
and rod-like towards the terminal plates. Colour in alcohol, light yellowish, whitish. 
Length about 55 mm. 
Habitat. — Kerguelen Islands; depth, 10 to 100 fathoms; a single specimen. 
The form of the body reminds one of that in Thyone raphanus, &c. The shorter 
dorsal surface and the posterior and anterior portions of the ventral surface have the 
pedicels about equally distributed, while those on the remaining middle part of the 
ventral surface are much more densely crowded. No arrangement of the pedicels in 
rows is discernible. The tentacles are small and completely retractile, consequently I 
have not been able to study their form and number ; it seems as if some of them are 
considerably smaller than the others. 
The calcareous ring is very small, only about 3 mm. in diameter, and its posterior 
undulating margin is devoid of any prolongations ; it is composed of ten simple pieces. 
Three Polian vesicles and a single madreporic canal are present. The retractors are 
attached somewhat in front of the middle of the body, and some of them are remarkable 
by being bipartite or tripartite towards their posterior ends ; moreover, the tripartite 
retractor is attached to three different longitudinal muscular bands, which doubtless is 
an abnormality. The reproductive organs consist of two small bundles of short simple 
tubes. The respiratory-trees are not very richly provided with short branches. The 
cups (PL V. fig. 7, 6) have the ends of the curved arms of the x -shaped central rod 
either simple or bipartite, in which latter case not four but eight holes are present ; they 
have a diameter of 0'056 mm. The plates (PL V. fig. 7, a) overlap each other and 
form a continuous layer under the cups ; they measure 0 '6 mm. in diameter, and are 
usually provided with many holes. They have a very irregular form. 
The species doubtless represents the Antarctic form of Thyone raphanus, but difiers 
from it mainly by the absence of anal teeth. 
