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



The actino-lateral spines, although long, do not meet in the interradium ; indeed the 

 abactinal and actinal membranes coalesce, apparently normally, in the outer portion of the 

 median interradial line, thereby forming a partition in the interradial chamber. The 

 spines which come near the interbrachial margin are much thickened and knobbed at 

 their extremity ; indeed all of them are more or less so except the most outward of all. 

 There are about thirty-two to thirty-six actino-lateral spines, the fifteenth to seventeenth 

 from the mouth being longest ; beyond this they diminish very rapidly in size. 



Colour in alcohol, white. 



Locality. — Station I. South-west of Cape Finisterre. December 30, 1872. Lat. 

 41° 58' 0" N., long. 9° 42' 0" W. Depth 1125 fathoms. Blue mud. 



Remarks. — Hymenaster membranaceus bears some superficial resemblance to 

 Hymenaster pullatus, but the forms may be readily distinguished by the character of the 

 supradorsal membrane, by the form and armature of the mouth-plates, by the shape of the 

 aperture-papillae, and by the structure of the web of the abactinal floor. 



20. Hymenaster coccinatus, Sladen (PI. XC figs. 3 and 4 ; PI. XCI. figs. 10-12). 

 Hymenaster coccinatus, Sladen, 1882, Journ. Linn. Soc. Lond. (Zool.), vol. xvi. p. 238. 



Marginal contour stellato-pentagonoid. Interbrachial arcs well rounded. Pays tapering 

 to a fine point, with their lateral margins almost straight. Minor radial proportion 55'5 

 per cent. R = 18 mm. ; r= 10 mm. Form very depressed, slightly convex and rising in 

 the centre. No definite marginal fringe present. 



The supradorsal membrane is very fine ; the muscular fibres are thin, filiform, and well 

 defined, forming a rather wide and rectangular reticulation resembling to a large degree 

 the venation of certain leaves more than the characteristic intercrossing of fibres radiating 

 from the neighbouring spinelets which is generally noticeable throughout the genus. 

 This peculiarity arises from the frequent bifurcations, bendings, and sudden terminations 

 to which the fibres are subject, which, together with the presence of small secondary 

 fibres, modify the normal arrangement of radiation from tip to tip, this being after all 

 the principle of the disposition of this structure even in the species under notice. The 

 meshes are filled in with an almost hyaline tissue, punctured with two, three, or even more 

 small spiracula, each surrounded with a definite white ring. The spinelets of the paxillse 

 are not numerous, and are but slightly protuberant, the tips being covered with a little 

 cap of membrane, which gives them a rather knobby appearance. The oscular orifice 

 is moderately large, its outer circumference at the base of the valves being well defined 

 by a pentagonal outline formed of thickened or fibrous tissue. The five valves are regular 

 and triangular, with about eight spines in each ; the whole series are webbed together, and 

 form a very slightly elevated pyramid when closed. 



The ambulacral furrows, which are wide and open, are very slightly petaloid opposite 

 the commencement of the outer third, and rather rapidly constricted towards the tip. 



