148 THE VOYAGE OF H.M.S. CHALLENGED 



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 cseruleus. The odontophore is exposed on the actiual surface, 

 having superficially the form of an eqmlaterally 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" E. Depth 2550 fathoms. Eed clay. Bottom temperature 34° - 5 Fahr. ; surface 

 temperature 56° - l Fahr. 



Remarks. — This small form is apparently nearly related to Porcellanaster cserxdeus 

 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 casruleus on PI. 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 Caulaster pedunculatus 

 has already been noticed, and it supports my strong conviction that the latter will 

 ultimately prove to be a young Porcellanaster. 



