IIS STROXGYLOCENTROTUS NUDUS. 



to an insignificant line of small secondary tubercles, similar in size to the 

 secondary tubercles separating the median ambulacra] space from the porif- 

 erous zone. In consequence of this arrangement of the tubercles and of the 

 poriferous zone the whole ambulacra! system is comparatively narrower above 

 the ambitus than in most species of the genus. In the interambulacral 

 region the tubercles are more nearly of uniform size; the difference in size 

 between the primary and secondary tubercles is not very marked. The 

 secondary tubercles occupying the rest of the coronal plates are not numer- 

 ous and irregularly scattered ; the miliaries are few in number. The test is 

 thick, the spines lorn;- and stout. The actinal surface is .somewhat flattened ; 

 the actinostome but slightly sunken, having large, broad cuts; actinal mem- 

 brane thin, thickly covered by long elliptical limestone platen Coronal 

 plates high, test somewhat turban-shaped, coloring of spines dark violet. 



\ , Width Porif Zone 



System, above Ambitus, on Act. Surface. 



5 8 3. 5.2 



Strongylocentrotus nudus 



! Toxocidaris nuda A. Ac, 18G3, Proc. A. X. S. Phila. 



! Strongylocentrotus nudus A. Ac, 1872, Rev, Ech., I't. I. ]>. 1G5. 



PI. XXIV. f. 26-30. 



Abactinal system large, nearly smooth ; one large secondary tubercle on 

 each plate, with here and there a miliary tubercle. Test thin, regularly 

 arched, somewhat globular. Only two vertical rows of large primary tuber- 

 cles in the median interambulacral space. Coronal plates high; one indis- 

 t i ii 't median vertical line of secondary tubercles, ami a row of smaller ones, 

 separating the median interambulacral space from the poriferous zone; small 

 secondaries loosely arranged round the scrobicnlar circles, lilling the rest of the 

 coronal plates; the same arrangement holds for the median anibulacral zone. 

 Poriferous zone narrow, pores arranged in nearly vertical arcs; in the median 

 anibulacral space two vertical rows of primary tubercles very much smaller 

 than those of the interambulacral region. Poriferous zone slightly petaloid 

 on actinal side. Actinostome large; actinal cuts slight ; spines long, slender, 

 tapering; test and spines colored dark violet-brown. Actinal membrane 

 thin, with few elliptical plates in prolongation of ambulacra. 



