284 THE VOYAGE OF H.M.S. CHALLENGER. 
the median line of height at right angles to the direction of the axis of the ray. Some- 
times only one spinelet is present, and this usually occurs in the interbrachial are; and 
sometimes a much smaller tubercle or granule may be present in addition to the two large 
ones above described, and this may stand either by the side of the upper spinelet or above 
it, in the latter case forming a lineal series of three along the median transverse line of 
the plate. 
The infero-marginal plates are similar to the superior series but are not all regularly 
correspondent, and their breadth is rather greater than that of the latter plates. Hach 
plate bears two or three spinelets or elongate tubercles, similar to those above described, 
placed in lineal series along the median transverse line of the plate; often one or two of 
these tubercles are much smaller. At the extremity of the ray the infero-marginal plates 
are smaller than the superior series, and only bear one spinelet. The surface of the 
marginal plates is slightly convex in the median transverse line, and the character is 
more or less emphasised by the membrane mounting the base of the spinelets. 
The adambulacral plates are large and broader than long. Their armature consists of 
three large, cylindrical, obtuse spinelets, which form a transverse series along the median 
line of the plate at right angles to the furrow. ‘The spinelet on the furrow margin and 
the middle one are subequal and much longer than any of the other spinelets on the 
animal ; the outer spinelet is smaller. These spinelets radiate slightly apart. Occasionally 
there is a second and smaller spinelet on the furrow margin on the adoral side of the 
furrow spine proper, and placed somewhat obliquely in relation to it. Occasionally there 
may be an additional small spinelet at the outer end of the transverse series. 
The mouth-plates are small and present no actinal prominence. Their armature 
consists of three spinelets on the free margin. The innermost spine is large and cylin- 
drical, the second is smaller and slightly tapering, the third is larger than the second and 
cylindrical, and should perhaps be counted with the superficial series. On the actinal 
surface of the plate are two much smaller spinelets, the outermost smallest, forming with 
the outer marginal spine a line of three, corresponding to the three spines on the adam- 
bulacral plates. No other spines are present on the plates. The plates do not appear to 
meet in the median suture, but no superficial indication of this is present, as the whole sur- 
face is covered with the same uniform layer of membrane as the other parts of the starfish. 
The actinal interradial areas are very small. Not more than four or six intermediate 
plates are present in each, and each of these bears a small tubercle similar to, but smaller 
than those on the abactinal plates. Consequently all except the innermost three or four 
adambulacral plates are contingent on the infero-marginal plates. 
The anal aperture, which is subcentral and slightly excentric, is conspicuous and 
margined by a number of robust elongate granules. 
The madreporiform body is small and has very coarse strize, which, in consequence of 
their disposition, give the organ almost the appearance of a group of coarse granules. It 
