20 THE VOYAGE OF H.M.S. CHALLENGER. 
centrally one, or sometimes two, and rarely three, very small papilliform thornlets, of 
uniform character throughout, excepting a few on the disk which are slightly larger, but 
still quite inconspicuous and unnoticeable without close examination. Occasionally in 
some examples two or three pedicellarian apparatus may be present,—these are of the 
double comb or spiracle-lke form; their structure is very simple, and the spinelets 
composing them are comparatively large, in fact, slightly larger than the small thornlets 
usually borne on the spicules. 
The supero-marginal plates, twenty-three to twenty-five in number from the median 
interradial line to the extremity, are elongate and low. The length is about three times 
greater than the height, and the upper margin of the plate, though actually confined to the 
lateral wall of the ray, is very slightly bevelled upon the abactinal surface. Each plate bears 
centrally on this margin a rather prominent tubercle, upon which is articulated an elongate, 
tapering, sharply pointed spine, the third or fourth from the interradial line measuring 
about 4°5-5 mm. in length. ‘There are generally one or two small thornlets near the base 
of the spine, otherwise the surface of the plate is simply covered with a very thin 
membranous tissue. In the median interradial line is a high, odd, supero-marginal plate, 
thick and tubercular abactinally, which bears an elongate, cylindrical, tapering spine, 
about 8 mm. in length, directed vertically. On the vertical wall of the plate which 
stands in the interbrachial are are a number of minute, conical, sharply-pointed granules, 
usually along the median area; and the one or two neighbouring plates on each side 
are likewise similarly, but irregularly and more sparsely, granulated. 
The infero-marginal plates are elongate and low like the supero-marginal series, to 
which they correspond. Hach plate bears a straight, tapering, lateral spine articulated 
on a tubercle, and usually more delicate and often shorter than the spine on the accom- 
panying supero-marginal plate. A second but much smaller spine is sometimes present 
on the inner side, usually in the larger examples, but it is often wanting altogether, and 
seldom appears on the inner part of the ray and towards the extremity. Occasionally 
one or two microscopic miliary thornlets are present on the plate, and the surface of the 
plate is covered with a delicate membranous tissue. 
In the median interradial line there is a large, broad, odd infero-marginal plate, larger 
and broader than any of the others; it is placed immediately beneath the odd supero- 
marginal plate. It bears a small representative of the lateral spine, and in addition 
several delicate miliary spinelets and thornlets. The one or two next plates on each 
side are likewise frequently more spinulate than any of the others. + 
The adambulacral plates are large in proportion to the size of the star fish, their length 
slightly exceeds their breadth, and they present a prominent and rather abrupt convexity 
into the furrow. ‘Their armature consists of :—(1.) a furrow series of five to seven small, 
short, cylindrical, obtusely tipped spinelets, subequal excepting the extremities of the series, 
which are rather shorter; all are closely placed and form compact isolated little combs. 
