GENEEA AND GEOUPS OF THE ECHINOIDEA. 193 



Ambulacra narrow, simikr, equal, straight, flush, widely open 

 to the ambitus, the parts of the poriferous zones with paired pores 

 widening and then diminishing to a point; pores beyond termi- 

 nating much before the margin of the test is reached, in single 

 series ; close to the peristome there is some doubling of pores 

 and a small phyllode. Ambulacral plates numerous, low, broad 

 primaries. 



Interradia large, with much fewer and taller plates than the 

 ambulacra. Tubercles of both areas similar, very small, per- 

 fcirate, crenulate, sunken in scrobicules, a miliary structure 

 between the scrobicules. 



Peristome small, central, pentagonal, wider than long ; phyl- 

 lodes nearly crowded out by bourrelets. Perignathic girdle re- 

 duced to small knobs upon the interradia ; jaws absent. Peri- 

 proct inframarginal, small, elliptical, transverse, close to the 

 posterior edge ; with three large plates and some small ones. 

 Spines short, slender, cylindrical, tapering. 



Becent. Caribbean Sea, 76-460 fathoms. 



The resemblance of the degraded perignathic girdle to that of 

 Echinoconus is very suggestive. 



GrenusPLESiOLAMPAS, Duncan ^ Sladen, 1882, Fal. Ind. ser. xiv., 

 Foss. Ech. W. Sind, pp. 9, 54, pis. i., xiii.-xv. (non Plesio- 

 lampas, Pomel, 1883, TMses, p. 62). 



The test is moderate in size, variable in shape, ovoid, elongate, 

 or polygonal, depressed, tumid at the margins and dorsum, con- 

 cave around the actinally placed peristome. 



Apical system excentric in front, with four basal plates, per- 

 forated and united ; madreporite in the right anterior, and ex- 

 tending into the others centrally, large and separating the 

 posterior lateral radial plates slightly. 



Ambulacra subpetaloid, widely open at the distal extremity ; 

 poriferous zones subequal, almost straight ; pores round, the 

 outer sometimes elongate. Poriferous zones continued on the 

 actinal surface as single pores ; a rudimentary phyllode. 



Interradia large and broad in comparison with the ambulacra ; 

 tubercles numerous, small, mamillated, imperforate, and non- 

 crenulated, in deep scrobicules ; miliaries numerous. 



Peristome central, subcentral, or excentric in front, sub- 

 pentagonal or oval-elliptical, transversely elongate, with a very 



LINN. JOUEN. — ZOOLOGY, VOL. XXIII. 13 



