600 THE VOYAGE OF H.M.S. CHALLENGER. 
separated, and with the interspace filled in with ligament. Their armature consists of a 
single large spine which stands perpendicularly, and is articulated on a tubercular elevation 
occupying the whole of the actinal surface of the plate. A spine near the base of the 
ray about midway along the ovarial inflation measures 4°5 mm. in length, is broad and 
robust at the base, tapers to the middle of the shaft, and then expands gradually into a 
flaring and compressed extremity, which is truncate and chisel-shaped. As they recede 
from the base of the ray, after passing the ovarial region, the breadth of the tip of the 
spine diminishes, and the chisel-like character is almost, if not quite, lost midway along 
the ray, where the spines still measure 4°5 mm., but are much more delicate. Traces of a 
membranous sheath with crowded microscopic pedicellarize are present on the outer side of 
a great number of these spinelets, but as to whether the spine was entirely invested I am 
unable to say. 
The actinostome is wide, its diameter being 15 mm. in a disk measuring 29 to 30 
mm. The buccal membrane is thick and plicate, becoming papillate at the mouth-margin. 
The mouth-plates, as well as the succeeding pair of adambulacral plates, which are appa- 
rently ankylosed, may be placed more or less obliquely or even perpendicularly and form 
a vertical margin to the actinostome. They are deeply excavated by the first pair of tube- 
feet, and this character is emphasised by the fact that lateral extensions proceed towards 
the adjacent pair and entirely bar the actinostomial entrance of the ambulacral furrow. 
The free margin of each plate which abuts on the buccal membrane is very slightly 
curved, and bears two or three short, robust, skin-covered mouth-spines, somewhat 
dog-tooth shaped, the outer two radiating away from the innermost one towards the 
furrow, and touching or interlocking with the corresponding spines of the adjacent mouth- 
angle, barring the entrance to the ambulacral furrow. One or two small isolated pedicel- 
lari may be present on these spines. Near the outer extremity of each plate is a single 
small, conical, skin-covered spinelet, which is directed over the furrow and separates the 
first and second pair of ambulacral tube-feet. No other spinelets are present on the 
mouth-plates. The width of the united pair of plates midway between their extremities 
is very narrow, and is less than the diameter of the adjacent tube-foot. 
The ambulacral tube-feet have large, centrally invaginated, terminal disks. 
The madreporiform body, which is convex and prominent, is situated at the extreme 
margin of the disk. Its surface is marked with fine, convoluted striations, rather widely 
spaced. 
Colour in alcohol, a bleached yellowish white, with a slight pinkish shade on the side 
of the ovarial regions, 
Localities. —“ Lightning” Expedition : 
Station 7. In the Faerée Channel. September 3, 1868. Lat. 60° 7’ 0” N., long. 
5° 21’ 0” W. Depth 500 fathoms. Bottom temperature 1°'1 C.; surface temperature 
10%5'C: 
