‘PORCUPINE’ DEEP-SEA DREDGING-EXPEDITIONS. 
739 
the third pair, remote from the others, penetrates the ambulacral plate near its outer 
end. 
About two thirds of their length from the outer edge, the ambulacral plates bend 
strongly towards the apical pole, as in the case of the interambulacral plates, in the 
direction contrary to that in which they overlap. The outer edges of the series of 
ambulacral plates are covered by the ends of the plates of the interambulacral series. 
Frequently only one ambulacral plate slips under the end of one interambulacral plate ; 
but perhaps once out of thrice the outer ends of two ambulacral plates approach one 
another, and together are covered by the end of one interambulacral plate. The ambu- 
lacral plates are thus considerably more numerous than the interambulacral. As already 
stated, the interambulacral and ambulacral plates do not differ materially either in their 
individual form or in their arrangement in combination on the opposing surfaces of the 
flattened corona. 
The primary tubercles are perforated. The mamelon is surrounded by a slightly 
marked ring, but is not crenulated. The areola is large and is not depressed, or very 
slightly so. The tubercle, mamelon, and scrobicule seem to be formed of a separate 
calcareous piece, cemented, as it were, to the surface of the plate which passes behind 
it, leaving the mamelon hollow, and a small conical space between the tubercle and 
the inner calcified layer of the plate. 
The larger spines (those which are articulated to the primary tubercles) are from 15 
to 20 millims. in length and '8 millim. in diameter. They are hollow as in Phormosoma , 
the fenestrated calcareous tube rising into from sixteen to twenty ridges, between which 
are uniform rows of fenestrse. As in the former genus, the ridges rise here and there 
into processes which have a distinctly spiral arrangement in the larger spines. Near the 
proximal end the spine is markedly enlarged, running out a kind of collar ; and from 
this point it diminishes, forming a regular cone down to the acetabulum, which is shallow, 
with a puncture in the centre for the insertion of the ligament of the spine (Plate LXIV. 
fig. 2). The fine spines articulated to the miliary tubercles are extremely delicate 
fenestrated calcareous tubes running to a fine point. 
The pedicellariae are very numerous and of three forms. The largest are the least 
numerous ; they occur in irregular rows parallel with and near the rows of large spines, 
and are also sparsely scattered, apparently irregularly, over the surface of the test. 
The head (Plate LXIV. fig. 4) is not less than 1-2 millim. in length ; and as these pedi- 
cellarise are attached to stalks as long as medium-sized spines, they can be seen quite 
distinctly with the naked eye (Plate LXIV. fig. 1). They have the general character 
of the tridactyle pedicellariae of the Echinoideans, only the basal triangular portion of 
each valve is unusually broad ; the middle part is arched outwards and narrow, and the 
terminal expanded part is shorter and more regularly spoon-shaped than in most species. 
The second form (Plate LXIV. fig. 5), which is a modification of the first more or less 
reduced in size and lengthened in its proportions, is like some of the common varieties 
in the Cidaridee. The third (Plate LXIV. fig. 6) resembles in all essential respects the 
