78 THE VOYAGE OF H.M.S. CHALLENGER. 



convex. The median suture is more or less imperfectly closed, and is widely open on the 

 outer part, sometimes partly exposing the odontophore, especially in the young form. 

 The armature consists of a marginal series of nine or ten small, cylindrical, short, 

 spinelets on each plate, the innermost two or three slightly longer and more robust ; 

 behind this is an irregular series of very short granuliform spinelets, the innermost, 

 however, being nearly as large as the innermost of the marginal series, with which they 

 form a group at the mouth-angle. The rest of the plate bears a few widely disposed 

 granules, more or less irregular in position, but with a tendency in some cases to form a 

 lineal series ; and the surface of the mouth-plate, as a whole, has the appearance of being 

 covered with widely spaced granules. 



The actinal interradial areas, though of small dimensions, contain a considerable number 

 of intermediate (or ventral) plates, above fifty being present in each area. These plates 

 are small, with a central, broad, low, but well-defined tubercular convexity, forming a 

 pedicle upon which are borne three or four small spinelets, constituting a very simple 

 paxilla. The plates form a subtransverse lineal series, running from the adambulacral 

 plates to the marginal plates, and four or five plates may be counted in the longest series 

 on each side of the median interradial line. No interradial plates extend beyond the 

 fourth or fifth marginal plate. 



The anal aperture is superficially indistinguishable in the material examined. 



The madreporiform body is large, compound, and more or less hidden by paxillae ; it 

 is only separated by a narrow space from the margin. 



Colour in alcohol, a bleached greyish white. 



Young Phase. — A young example, which measures E^IS'5 mm.; r-4 = 4"5 mm., 

 is readily recognisable even at this early stage as belonging to the species. The rays, 

 however, have not cpiite the attenuate character of the adult, for though tapering they 

 have an obtuse appearance, and the terminal plate is large and broad, whilst in the fully 

 grown form it is more elongate. There are fourteen supero-marginal plates, counting from 

 the median interradial line to the extremity; and they form a comparatively broader 

 border on the abactinal surface. The minuteness of the paxillas on the abactinal surface 

 is remarkable. The anal aperture is distinguishable. The madreporiform body is very 

 near the margin, and a large plate stands on its adcentral side. I have been unable to 

 detect any papula\ The adambulacral plates are very long and narrow, and their arma- 

 ture is already essentially the same as that of the adult. There are five or six spinelets in 

 the slightly curved furrow series, and a longitudinal series of four or five behind these 

 form a straight line parallel to the furrow. The mouth-plates are remarkably long and 

 narrow. The actinal interradial areas are very small, and not more than ten to twelve 

 minute plates are present in each. The lateral spinelets, or rather their incipient representa- 

 tives, have a tendency to be placed near the aboral margin of the infero-marginal plates. 



Locality. — Station 79. Between the Azores and Madeira. July 11, 1873. Lat. 



