148 THE VOYAGE OF H.M.S. CHALLENGER. 
portion occurs in that form. Furthermore, the height of the infero-marginal plates 
diminishes as they proceed along the ray, and the outermost plate has the shape of a 
thin elongate wedge. There are only three infero-marginal plates between the median 
interradial line and the extremity, and there is no trace of an infero-marginal plate 
below or corresponding to the small supero-marginal plate next the terminal. The 
vertical sutures between the infero-marginal plates do not fall exactly below or in line 
with the sutures of the superior series. ‘The terminal plate is extraordinarily large, and 
elevated conically towards the extremity. On this elevation is borne a vertical spine. 
At the extremity of the plate are two similar spines, one on each side of the median 
line, directed horizontally and radiating slightly apart. 
The adambulacral plates are large, and already show the characters of an adult Porcel- 
lanaster ; it may be noted, however, that they appear to be set somewhat obliquely. Each 
plate bears two spinelets on the furrow margin, side by side, and radiating a little apart. 
The spinelets are short, but broad and flattened, and rather leaf-like or lanceolate in form. 
The mouth-plates are markedly coulter-shaped, the adjacent edges of each plate in a 
pair being turned up vertically, forming a high-crested keel, and the suture is not closed, 
but often gaping more or less. The armature consists of a single spinelet at the junction 
of the two plates and two spines on the margin of each plate, resembling in this respect 
the adult of Porcellanaster cewruleus. The odontophore is exposed on the actinal surface, 
having superficially the form of an equilaterally triangular wedge which fits in the angle 
formed by the aboral and unclosed ends of the two mouth-plates. 
The actinal interradial areas are covered with an extremely delicate semi-transparent 
membrane in which no trace of actinal intermediate plates is yet to be detected. 
Locality.—Station 137. In the South Atlantic, midway between the Island of Tristan 
da Cunha and the Cape of Good Hope. October 23, 1873. Lat. 35° 59’ 0”S., long. 
1° 34’ 0” KE. Depth 2550 fathoms. Red clay. Bottom temperature 34°'5 Fahr. ; surface 
temperature 56°°1 Fahr. 
Remarks.—This small form is apparently nearly related to Porcellanaster cxruleus 
of the North Atlantic, but may be distinguished by the more elongate and attenuate 
rays, even when compared with a rather larger young example of that species. Except- 
ing these characters and the purely embryonic structures described above, the figures 
of the young phase of Porcellanaster cxruleus on Pl. XX. figs. 6, 7, represent the 
general aspect of the young form under notice so well that I have not considered it neces- 
sary to give additional drawings. For the general proportions and composition of the 
abactinal area the woodcut on the preceding page will supply what is requisite. 
The striking manner in which the general structure of this young example agrees 
with the description given by Perrier of the form he has named Cawlaster pedunculatus 
has already been noticed, and it supports my strong conviction that the latter will 
ultimately prove to be a young Porcellanaster. 
