140 THE VOYAGE OF H.M.S. CHALLENGED 



with five spines, and by the clavate spinelets of the disk. This species is especially 

 remarkable for the length of the tubular epiproctal prolongation or anal funnel. 



3. Porcellanaster tuberosus, Sladen (PI. XXIII. figs. 1-4 ; PI. XXVII. figs. 13-16). 



Porcellanaster tuberosus, Sladen, 1883, Journ. Linn. Soc. Lond. (Zool.), vol. xvii. p. 223. 



Pays five. R = 18*5 mm. ; r — 6 mm. R = 3 r. 



The rays spring gradually from the angles of the disk and taper moderately towards 

 the extremity, maintaining a robust character throughout ; the minor radius is in the 

 proportion of 32 per cent. The disk is not high, and very slightly inflated. The 

 interbrachial arcs are well rounded. 



The abactinal area is covered with a rather fleshy integument beset with simple 

 spinelets somewhat closely placed ; these are short, cylindrical, obtuse, covered with 

 membrane, and occupy the whole of the surface excepting only the extreme angle 

 at the base of the ray. A well-developed epiproctal tubular prolongation rises from 

 the centre of the abactinal area, and is nearly equal in length to the distance between 

 the centre and the inner edge of the marginal plates in the interbrachial arc ; it tapers 

 very slightly towards its extremity, and is indurated with spicular spinelets like the 

 rest of the abactinal membrane. 



The marginal plates form a deep margin and curve over roundly in the interbrachial 

 arcs, the inferior as well as the superior series being visible from above. Upon the 

 rays the superior series arch well over and almost meet in the median dorsal line, giving 

 to the ray a more or less subcarinate character. The supero-marginal plates are four 

 in number from the median interradial line to the extremity, exclusive of the large 

 terminal plate, and all are distinctly longer than high. The second and third supero- 

 marginal plates from the median interradial line bear short, conical, upright spinelets; 

 but all the rest are unarmed excepting the terminal plate, which carries three spines — one 

 at the extremity in the median line of the ray, and one on each side at the anterior 

 extremity of the inferior margin of the plate. The terminal plate is swollen and 

 prominently tubercular abactinally, and is excavated on its outer extremity for the 

 passage of the terminal ambulacral tube. In one ray of the specimen under notice, 

 the penultimate supero-marginal plates are also swollen and ankylosed in such a manner 

 as to resemble the terminal plate, and bear a single spinelet. 



The infero-marginal plates, which are five in number, are much shallower than the 

 superior series, and also shorter. The two series consequently do not correspond, a result 

 probably brought about by the extreme development of the terminal plate, which occupies 

 the space of both superior and inferior plate. 



One cribriform organ is present in each interbrachial arc ; it is rather broad and has a 

 deep depression down the median line. The structure is lamelliform. (See PI. XXVII.) 



