746 
PEOFESSOE WYVILLE THOMSON ON THE ECHINOIDEA OE THE 
the test is the thinnest (Plate LXIX. fig. 3) ; the outline rises to the apex and then 
sinks gradually to the truncated posterior rostrum, along the top of which it coincides 
with a slight longitudinal ridge. The oral surface of the test also rises slightly from 
the anterior border to produce the depression in which the mouth is placed, and from 
the mouth it sinks towards the truncated extremity forming the floor of the rostrum. 
This truncated end is occupied by a deep inversion (Plate LXIX. fig. 4), deepest 
above, at the bottom of which the anal orifice opens. The periproct is oval, large, and 
plated with small scales. In one specimen there is no trace of the exserted anal tube 
described by Prof. A. Agassiz as occurring in the specimens dredged by Count Pouktales 
in the Strait of Florida. 
The ambulacra have all precisely the same character. Those of the bivium are appa- 
rently in slight depressions. This is, however, only an effect produced by the slight 
projection of the sides of the posterior rostrum (Plate LXIX. fig. 4). The ambulacra 
are not very easily seen, the pores are so minute ; by holding the shell up to the light, 
however, they become sufficiently apparent as rows of simple pores passing between 
irregularly hexagonal plates, in double series, from the apex to the mouth. The ambu- 
lacral arese widen somewhat from the apex to the ambitus, and become slightly narrower 
from the ambitus towards the mouth ; they are about 4 millims. in width at the widest 
point, the lateral interambulacral spaces being 12 millims. At the mouth the ambu- 
lacra expand into a very distinct floscelle (Plate LXIX. fig. 6), and the interambulacral 
arese end in bourrelets crowded with tubercles and bearing combs of long spatulate 
spines. The plates of the apical disk are so compacted and fused together that it is 
difficult to trace their outline. Eight holes, nearly of equal size, surround a central 
madreporic tubercle (Plate LXIX. fig. 5) ; of these, five terminate the ambulacra and 
are the pits for the sense-organs, the other three are ovarial. The posterior and the right 
anterior ovaries and ducts are undeveloped. 
The surface of the test is crowded with minute tubercles for the articulation of the 
larger spines and many small granules (Plate LXIX. fig. 5). The tubercles are imper- 
forate, with a smooth mammillary boss ; they are placed in circular scrobicular depres- 
sions, but they project somewhat above these and above the surface of the test. The 
larger spines, articulated to the tubercles, are cylindrical, fenestrated, with slight aspe- 
rities on the longitudinal calcareous shafts (Plate LXIX. figs. 7, 8, 10). 
The small spines, which, attached to the minute granules, form a close underfelting 
all over the test, are fenestrated and slightly roughened, and expand at the end into a 
rosette of pointed tubercles (Plate LXIX. fig. 9). The pedicellariae, articulated to some 
of the granules, are three-valved ; they are very small and of a somewhat peculiar form, 
though resembling generally the smallest pedicellariae in Echinus. The bases of the 
valves are wider, the valves themselves are rounder and more arched and toothed round 
the edge (Plate LXIX. fig. 11). Pound the mouth there are groups of three and 
four very small pedicellariae, differing in form somewhat from the others, and with the 
bases of the valves apparently fused together (Plate LXIX. fig. 12). 
