748 
PBOEESSOB WYVILLE THOMSON ON THE ECHINOIDEA OE THE 
then passes up the centre of the anterior end of the test to the apex (Plate LXXI. fig. 3). 
The two lateral ambulacra of the trivium pass from the sides of the peristome upwards 
over the sides of the shell, just behind the lateral borders of the anterior end, to meet 
at the same point (Plate LXXI. fig. 3). The ambulacra of the bivium start very close 
together from this lower end of the mouth (Plate LXXI. fig. 4) ; they pass along on 
either side of the ventral plastron to a little beyond the anus, nearly to the end of the 
shell ; then they turn upwards and, still separated by the interambulacrum of the 
bivium, run forwards on the dorsal aspect of the test to the secondary or posterior pole 
of the apical system (Plate LXXI. figs. 6, 8). The tube feet in connexion with the 
odd ambulacral vessel of the trivium are conical and of considerable size ; those springing 
from the other four ambulacral vessels are uniform throughout, very small and pointed. 
The interambulacral areas of the trivium are comparatively narrow, and form the 
lateral portions of the anterior truncated extremity running up to the principal apical 
pole. The lateral interambulacra are wide, and correspond with the space between the 
primary and secondary apical poles. The odd interambulacrum of the bivium is most 
remarkably developed, and chiefly contributes to the peculiar form of the shell. Start- 
ing from below the mouth, it forms a narrow plastron along the middle line of the 
ventral surface of the test ; it then widens out to form the posterior rostrum, arching 
upwards to the edge of the periproct; it forms the projecting arch over the anal open- 
ing, whence it continues along the median ridge of the back to end with the bivial 
ambulacra in the secondary apical pole (Plate LXXI. figs. 1, 2, & 3). 
The apical system is disjunct, as in Dysaster. The anterior portion consists of a series 
of coalesced ovarial plates, with four large and distinct apertures, the two anterior closer 
to one another than the two posterior, all of them inclining markedly forwards in passing 
through the test (Plate LXX. fig. 4). The ocular plates are undistmguishable, and 
there are no evident ocular pores. The posterior pole of the apical system consists of 
two odd plates terminating the ambulacra of the bivium, and therefore representing 
ocular plates. These plates show no pores, and they are so small and indistinct that 
this “ secondary pole ” is chiefly indicated by the junction of the bivial ambulacra. 
The anal opening is small ; the periproct is oblong, transverse, and plated with irre- 
gular unequal calcareous scales (Plate LXXI. fig. 7). 
The surface of the test is somewhat sparsely and irregularly sprinkled with primary 
tubercles ; the scrobicular area is rounded or slightly angular, roughened and slightly 
depressed ; the mammillary boss is low, and peculiarly crenulated by a chain of minute 
rounded elevations. The tubercle is perforated, and likewise slightly crenulated 
(Plate LXX. figs. 6, 7). Between the primary tubercles there are numerous minute 
tubercles for the articulation of the small spines and the pedicellariae. The larger 
tubercles are specially crowded near the apex and round the border of the anterior 
flattened surface (Plate LXX. figs. 1, 2). 
The primary spines are cylindrical, fenestrated, with about twelve smooth ribs ; the 
milled ring is very prominent, and gives attachment to a circle of very strong and 
