ASTBEOIDEA OF H.M.S. ' CHALLENGE!! * EXPEDITION. 231 



the ray ; the aboral spinelet is much smallei' than the other two 

 of which the adoral is slightly the longest. Aperture-papillfB 

 sraall and oval or subcircular in form, sometimes expanded late- 

 rally to snch an extent that the breadth is greater than the 

 length. 



Mouth-plates comparatively small and short ; keel prominent, 

 having a rhomboid outline when seen from above, and inclined 

 upwards into the mouth-cavity, with rather widely 'expanded lateral 

 flanges, straight and square in front. Each plate bears two robust 

 secondary spines, one on the middle of its surface, standing in 

 the lateral angle of the rhomb, and another, comparatively smaller 

 and thinner, placed near the adoral extremity. Mouth-spines 

 proper are represented by three small tapering spinelets placed 

 on the lateral margin of each plate. 



Actino-lateral spines robust and of moderate length, the 7th 

 to 9th from the mouth being longest. The spines of two adjacent 

 rays do not quite meet in the median interradial line, a little 

 narrow channel or wrinkle of the membrane being maintained 

 between their tips. The spines diminish regularly in length after 

 the angle is passed, until they become microscopic at the end of the 

 ray ; they are pointed at their outward extremity ; and the web 

 being well indented between gives a serrate appearance to the 

 margin. 



Station 157. Lat. 53° 55' S., long. 108° 35' E. Depth 1950 

 fms. ; diatom-ooze. 



Htmenaster poeosissimus, n. sp. 



Marginal contour subpentagonal ; interradial angle scarcely in- 

 dented, forming simply a slight curve inward. The lesser radius 

 is in the proportion of 75"5 per cent.; i2=45 millim., r=34 

 millim. The radii do not taper beyond the extent of a true 

 pentagon, and are slightly rounded at the extremity, which is 

 somewhat feebly upturned. 



Supradorsal membrane very uniformly reticulated; the muscular 

 fibres so closely and regularly placed that their radiation from the 

 spinelet-tij)S as centres is scarcely apparent ; meshes very small 

 and regularly spaced, each with one small spiraculum. Paxil lae- 

 spinelets 3, 4, or 5 in number, evenly spaced, rounded at the tips, 

 very slightly prominent, and producing a unifoi'mly papillate 

 . appearance on the dorsal area, no general pattern of arrangement 

 being discernible. The crowns of a great number of the paxillge 



LINN> JOURN. — ZOOLOGY, VOL. XVI. 16 



