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PROFESSOR WYYILLE THOMSON ON THE ECHINOIDEA OP THE 
Porocidaris purpurata, Wy. T. (Plate LXI. & Plate LIX. figs. 14 & 15.) 
Synonyms : — Porocidaris purpurata, Wyville Thomson, Proc. II. S. 1869, 1872 ; The 
Depths of the Sea, 1873; Porocidaris purpurata, Alexander Agassiz, Kevision of the 
Echini , 1872. 
The test (Plate LXI. figs. 2, 3), in what appears to be a full-sized example of this 
species, from a depth of 542 fathoms about 100 miles to the north of the Butt of the 
Lews, is generally globular, but decidedly pentagonal in outline, owing to the central 
portions of the interambulacral arese projecting considerably beyond the lateral portions 
and the ambulacra. It is 50 millims. in diameter at the ambitus, and greatly depressed, 
its polar axis not being more than 35 millims. in length. The ambulacra are narrow 
and band-like, 4 millims. in width between the outer edges of the pore-arese, which 
occupy the greater part of the space, seeing that the central miliary strap of the ambu- 
lacra bears only two marginal rows of larger miliary tubercles with a few minute granules 
irregularly scattered between them. 
The interambulacral arese are wide, about 22 millims. in width at the ambitus. The 
plates are large, eight or nine in longitudinal series. The areolae, which are transversely 
elliptical, slightly depressed, and surrounded by a raised border, occupy the greater part 
of the surface of the plates, the interspaces being crowded with miliary granules. A 
single row of large granules occupies the raised border of the areola. The bosses for 
the primary tubercles are rather prominent, those on the oral aspect of the test smooth, 
those on the apical aspect strongly crenulated, the change taking place quite suddenly 
at the ambitus. The tubercles are of moderate size, smooth, and perforated. 
The peristomial opening in the test is proportionally rather small, in the specimen 
described 7 millims. in diameter. The 
peristome is subpentagonal, not notched. 
The buccal membrane is paved with im- 
bricated scales, ten double rows ambu- 
lacral, and perforated up to the edge of 
the mouth with double pores in conti- 
nuation of the pore-areas of the corona. 
The interambulacral spaces are filled up 
by a single row, or an imperfect double 
row, of imperforate scales, which alter- 
nate in imbrication with the neighbouring 
ambulacral scales. The free edges of all 
the scales are crowded with granules for 
the articulation of small spines of a special 
form. On the inner surface the ambu- 
lacral scales of the buccal membrane rise Auricles and buccal membrane of 
up into a series of curved spines on each Porocidaris purpurata. x 5. 
side of the ambulacral vessel, so as to form a kind of channel for the passage of the 
