TETRAPYGUS NIGER. 73 



The only species of this genus known is the common littoral urchin of 

 Chili and Peru, to which Molina gave the name Echinus niger. The peculi- 

 arities of the ambulacra are evident even in very small specimens. 



Tetrapygus niger Agass. 



Echinus niger IMolina, 1782. Saggio St. Nat. Chili, p. 175. 

 Tetrapygus niger Agassiz, 1846. Ann. Sci. Nat., VI, p. 354. 



Plate 47, figs. 12-lG. 



The tridentate pedicellarisj are small and scarce, and are found only on 

 the actinal surface. The valves (fig. 12) are more slender than in A. 

 Ditfresuii, and are distinctly constricted near the middle. They measure 

 from .25 to .85 mm. in length, while the stalks on which they are borne are 

 5-15 times as long. 



The ophicephalous pedicellariae are abundant everywhere, particularly 

 on the buccal membrane. They are very characteristic, for the valves 

 (fig. IJf) are usually flattened or even concave at the tip, and have a broad 

 and deeply grooved apophysis, while the head of the stalk (fig. 13) is 

 swollen but contracted at the tip. The valves measure from .25 to .75 mm. 

 in length, and the stalks are 3-20 times as long. Intermediate forms be- 

 tween tlie ophicephalous and tridentate pedicellaria3 are not uncommon. 



The triphyllous pedicellariae are very scarce, and are not peculiar. The 

 heads measure .35 mm. in length, and the stalks are about nine times as 

 long. All the pedicellariae are purple in color in adult specimens. 



There seem to be no calcareous particles in the pedicels, except the usual 

 terminal plates and their supporting rods in those of the actinal surface, but 

 the plates in the gills are numerous and large, with from 10-60 perfora- 

 tions. Similar but larger plates occur in the buccal membrane. 



The sphreridia (figs. 15, 16) in young individuals are approximately 

 globular, but in large specimens they are wider than long, and may even 

 become somewhat any:ular. 



■£3" 



PODOCIDARIS. 



A. Agassiz, 1869. Bull. M. C. Z., I, p. 258. 



Type-species, Podocidaris sciilpta A. Agassiz, 1869. Bull. M. Z. C, I, p. 258. 



The establishment of Pygma^ocidaris and Habrocidaris leaves this a 

 monotypic genus, containing only the species which has been taken a 



