368 THE VOYAGE OF H.M.S. CHALLENGER. 
B. Bathymetrical range: 5 to 658 fathoms. 
y. Nature of the Sea-bottom: Rhegaster tunudus occurs on Clay, Blue clay, and 
Hard ground. The bottom inhabited by Rhegaster murray: is not recorded. 
Chorological Synopsis of the Species. 
Ocean. Range in Fathoms. Nature of the Sea-bottom. 
Rhegaster murrayt . : Atlantic. 285 to 433 
Rhegaster tumidus . ‘ Atlantic. 5 to 658 Clay, Blue clay, Stony ground. 
1. Rhegaster murrayi, Sladen. 
Rhegaster Murrayi, Sladen, 1883, Trans. Roy. Soc. Edin., vol. xxxii. p. 156, pl. xxvi. figs. 2-7. 
Marginal contour subpentagonal ; rays slightly produced ; the minor radius in the pro- 
portion of 77 per cent., oras 5:6°5. R=14'3 mm.; r=11 mm. 
Interbrachial ares somewhat indented at the median interradial line, from whence the 
contour curves outward faintly, consequent on a slightly tumid swelling at the base of the 
ray, and is then gracefully incurved towards the tip, which is obtuse and rounded. Abac- 
tinal area high and convex over the disk, sloping down regularly to the extremity of the 
rays, the height at the centre of the disk being 11°75 mm. A feeble sulcus or depression 
is present on the outer part of the median interradial line, which emphasises the tumid 
character of the base of the rays. Actinal surface more or less flat, excepting that the 
rays are slightly turned up at their extremity, and that a rather sharp depression occurs 
in the interradial areas along the inner part of the median interradial line, behind the 
mouth-plates. 
The abactinal area is covered with short delicate spinelets, all of uniform length and 
size, their lower portion being apparently sunken in membrane. ‘The spinelets stand 
perpendicularly, and are closely placed, presenting to the naked eye the appearance of a fine 
and uniformly granular surface. When magnified the spines are seen to be slightly 
expanded or flaring outwardly, and to be composed of many rods or lamelle, with the 
extremity of each individual lamella terminating in a short thorn-like point. This spinous 
abactinal area is punctured with numerous small but conspicuous pores, which are 
irregularly distributed at small but unequal distances apart over the whole area, except- 
ing the extremities of the rays and a narrow band along the median interradial 
line; towards the margin the apertures are smaller, wider apart, and less frequent. 
Through these apertures the papule are protruded, and under magnification a small 
but definite circlet of the abactinal membrane surrounding the puncture of the papula, 
and unencroached upon by spinelets, may be seen. No grouping of the abactinal 
