3G8 



THE VOYAGE OF H.M.S. CHALLENGER. 



/8. Bathymetrical range: 5 to 658 fathoms. 



y. Nature of the Sea-bottom : Rhegaster tumidus occurs on Clay, Blue clay, and 

 Hard ground. The bottom inhabited by Rhegaster murrayi is not recorded. 



CJwrological Synopsis of the Species. 



1. Rhegaster murrayi, Sladen. 



Rhegaster Murrayi, Sladen, 1883, Trans. Koy. Soc. Edin., vol. xxxii. p. 156, pi. xxvi. figs. 2-7. 



Marginal contour subpentagonal ; rays slightly produced ; the minor radius in the pro- 

 portion of 77 per cent., or as 5 : 6*5. E= 14 - 3 mm. ; r= 11 mm. 



Interbrachial arcs 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 1175 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 lamellae, 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 papulae 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 



