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



or subpapilliform granules, except at the adoral and aboral margins, where they become 

 more or less cilia-like. The suture-lines between the plates are well defined. 



The infero-marginal plates correspond exactly to the superior series, their line of 

 union being perfectly horizontal. About midway along the ray the height is subequal to 

 the length, but increases in proportion in the inner part of the interbrachial arc, and de- 

 creases at the extremity of the ray. The breadth on the actinal surface is subecpial to the 

 length, excepting at the base of the ray and along the interbrachial arc, where it is greater 

 than the length. The union of the actinal and lateral planes is well rounded. Each plate 

 normally bears a single, very minute, conical thornlet, on the rounded angle ; within the inter- 

 brachial arc these are placed centrally, but along the ray, where they are greatly reduced 

 in size, they stand nearer the aboral margin of their plate, and they point towards the 

 extremity of the ray at so acute an angle as almost to appear appressed to the lateral wall. 

 Furthermore they are so small as only to be noticeable with the aid of a magnifying glass. 

 The longest, which are in the interbrachial arc, are little more than half a millimetre in 

 length. The surface of the plate is covered with rather large hemispherical granules 

 distinctly spaced, which become subpapilliform or cilia-like and crowded at the adoral 

 and aboral margins. The sutures between the plates are well defined ; and those in the 

 interbrachial arc are especially deeply incised, recalling the structure of Astropecten. 



The adambulacral plates are long and narrow, with the furrow margin slightly convex. 

 Their armature consists of: — (1.) A furrow series of eight moderately elongate, cylin- 

 drical, and faintly subclavate spinelets — the length diminishing slightly towards each 

 extremity of the series, but the most aboral spinelet being usually very small. They 

 radiate so very little apart, that their position might almost be described as parallel to one 

 another. (2.) On the actinal surface of the plate immediately behind the furrow series is 

 a parallel row of five or six short papilliform spinelets, less than half the length of the 

 furrow series, even where longest on the inner part of the ray, whilst on the outer part of 

 the ray they are dwarfed into little more than granules. External to this are two lineal 

 series of granules filling the rest of the surface of the plate. In these, however, there is 

 sometimes a little irregularity, which is apt to give a more or less grouped appearance ; 

 and two or three additional granules may occur, especially on the inner part of the ray. 

 The eight or ten outermost adambulacral plates at the extremity of the ray have a single 

 comparatively robust conical spine, immediately behind the furrow series, much larger 

 than the other spinelets on the plate. Although these are not noticed without a magnify- 

 ing glass they are remarkable, as no trace of them is to be found on the other adambulacral 

 plates throughout the ray. 



The mouth-plates are elongate and narrow, the united pair being elliptical in outline, 

 slightly convex actinally, and with the median suture imperfectly closed, especially 

 inwardly, where it is often open and gaping. Nearly half their length is free, protruding 

 over the actinostome. The armature consists of a marginal series of nine or ten mouth- 



