CHAP, III.] THE CRUISES OF THE ‘PORCUPINE, 109 
The shell is singularly unlike that of any other 
known living echinoderm. It is about two inches in 
length, almost cylindrical, ending posteriorly in a 
blunt rostrum, and the anterior extremity is truncated. 
The surface of the shell is covered with short 
spatulate spines, and near the anterior end there is 
a kind of fringe of long thin cylindrical spines, espe- 
cially congregated on the upper surface. The mouth 
is at the bottom of a deep anterior and inferior groove, 
and the excretory opening is at the bottom of a pit 
on the dorsal surface, above the terminal rostrum. 
The arrangement of the ambulacra is most peculiar. 
The four ovarial openings and the madreporic tubercle 
are on the dorsal surface, just above the truncated 
anterior end at the base of which the mouth lies, 
and the three ambulacral vessels of the ‘trivium’ take 
a short course from the oral vascular ring, one along 
the centre of the anterior face, and the other two 
round ‘its edges to meet in a ring surrounding the 
ovarial openings. The two vessels of the ‘bivium’ 
have a very singular course. They run back into 
the great posterior prolongation of the shell, on the 
sides of which they form long loops, sending conical 
water-feet through single pores in long double lines of 
somewhat irregularly-formed ambulacral plates, which 
finally converge in a point a considerable distance 
behind the point of convergence of the three am- 
bulacra of the bivium. Between these two points of 
convergence, which are both on the middle line of 
the back, several plates are intercalated. We have 
thus the three anterior ambulacra ending in their 
ocular plates, meeting at one point, where there 
are likewise four genital plates, and the madreporic 
