REPORT ON TIIE ASTEROIDEA 507 



8. Hymenaster vicarius, Sladeu (PL LXXXIV. figs. 3 and 4; PL LXXXVI. 

 figs. 1-3). 



Hymenaster vicarius, Sladen, 1882, Journ. Linn. Soc. Lond. (Zool.), vol. xvi. p. 223. 



Marginal contour subpent.igonal. Interbrachial arcs well indented, the minor radius 

 being in the proportion of 69*2 per cent. R = 39 mm. ; r = 27 mm. The lateral fringe is 

 more or less distinct, and its margin forms a line that curves outwards after passing the 

 middle of the ray, adding greatly to the appearance of its breadth ; the margin then 

 suddenly contracts and emphasises the rapid tapering of the ray at the extremity. 



The supradorsal membrane is thin and papyraceous in appearance. The muscular 

 fibres are numerous and irregular, rather thick, closely placed, and radiate from the 

 tips of eac*h spinelet to those around, thus forming an interlacing web with moderately 

 large meshes of rhomboid or quadrate form. The interspaces are filled in with transparent 

 membrane, punctured in the centre by a spiraculum ; these are moderately large, well 

 spaced, and uniformly, though irregularly, distributed over the abactinal area. The 

 paxillse are numerous, and composed of four, five, or six spinelets, rather short and 

 pointed, their tips elevating the membrane into small pointed papillae, which are very 

 evenly spaced over the abactinal area and appear to rise sharply from the surface, 

 whilst their small size and comparative closeness of position give an easily recognisable 

 character to the starfish. 



The ambulacral furrows are moderately broad, lanceolate in outline, and taper rather 

 rapidly before they reach the extremity, which is in consequence somewhat produced. 

 The armature of the adainbulacral plates consists of two rather short, cylindrical, and 

 pointed spines, covered with an extensive investing membrane which forms an elongate 

 sacculus at the tip, and also adds greatly to the apparent robustness of the spinelets. 

 The aperture-papillas are large, acumino-spatulate, wide and rotund at the base, and often 

 contract rapidly to a point at the free extremity, or may be simply rounded. 



The mouth-plates are small, elongate, narrow, keeled, and prominent aborally. Each 

 bears two short robust secondary surface-spines, one near the adoral extremity, and the 

 other placed more aborally near the middle of the plate. The horizontal margins of the 

 plate falling into the actinostome are expanded into a rather wide flange, upon the 

 edge of which are situated four or five short cylindrical mouth-spines, slightly curved and 

 rounded at the tips, but not tapering. 



The actino-lateral spines are delicate, those included within the disk being short and 

 not reaching up to the median interradial line, but leaving a rather wide space. The 

 longest spine is about the fifteenth from the mouth, but even these do not meet in the 

 interbrachial arc, where a considerable amount of thickening and deformity of the mem- 

 brane occurs. The tips of the spinelets are rather tapering, and not thickened or knobbed. 

 The margin of the web is slightly thickened, not indented or festooned. 



