EEPOET OiST THE HOLuTHURIOIDEA. 
95 
Genus Thyonidium, Dliben and Keren, 1844. 
Thyonidium cehuense, Semper (PL IX. fig. 4). 
Habitat. — Station 201, October 26, 1874; lat. 7° 3' N., long. 121° 48' E.; depth, 
8 2 fathoms ; stones and gravel ; one specimen. 
This specimen, which is 1 1 mm. long, is ovate and of a light yellowish -brown colour. 
The pedicels are comparatively long and their sucking-disks well marked ; they seem to 
be disposed in five longitudinal series, but from the contracted state of the animal the 
arrangement is not very clear. The specimens examined by Semper have an arrange- 
ment of the pedicels in rows which is distinct only in the posterior extremity of the 
body, but it must be kept in mind that the Challenger specimen is younger, and that 
young forms are often characterised by having the pedicels more distinctly arranged in 
rows. I only counted nineteen tentacles. A single Polian vesicle and madreporic 
canal are present. The five interradial pieces of the calcareous ring (PI. IX. fig. 4, a) 
are simple ; each of the radial, on the contrary, consists of a larger main-piece and two 
posterior prolongations, each composed of two or three joints. Semper seems to have 
found only one kind of tables ; in the Challenger specimen, however, I have observed, 
in addition to the small tables, large ones scattered among the former. The small tables 
(PI. IX. fig. 4, h, c, d) have a rounded disk with the margin undulated and regularly 
perforated by eight peripheral holes and a slightly larger central hole ; the diameter of 
the disks measures 0’08 mm. The sj^ire of these tables, when fully developed, consists 
of four rods and one transverse beam, and terminates in about eight small teeth. Often, 
however, the spire is more or less incompletely developed. The larger tables (PI. IX. 
fig. 4, e) have the disk more irregular and the spire mostly devoid of teeth ; the disk 
measures 0T6 mm. in diameter or more. The pedicels are strengthened by deformed 
tables which have the disks elongate and rod-like, and the spire more or less irregular ; 
their shape will be best understood from the figures (PI. IX. fig. 4, f). 
Since some small differences exist, and since, moreover, Semper’s description of the 
calcareous deposits is very unsatisfactory, I cannot be fully convinced of the identity 
of the Challenger specimen with those examined by Semper. 
Thyonidium rugosum, n. sp. (PL V. fig. 5). 
Body tapered posteriorly into a conical caudal portion. Tentacles eighteen, five 
small pairs alternating with three pairs of large tentacles and two unpaired large ones. 
The smaller tentacles several times smaller than the larger. Pedicels very closely 
crowded, and distributed over the ambulacra as well as the interambulacra ; no arrange- 
