CHAP, in.] 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, especi- 

 ally 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 



