ARBACIA DUFRESNII. 69 



Arbacia Dufresnii Gray. 



Echinus Dufresnii Blainville, 1825. Diet. Sci. Nat., XXXVII, p. 76. 

 Arbacia Dufresnii Gray, 1835. Proc. Zool. Soc. London, p. 38. 



Plate 47, figs. 1-11. 



The tridentate pedicellariae (fig. 1) are rather infrequent and occur chiefly 

 on the actinal surface, though some occur at the ambitus. They appear to 

 be wanting in young individuals. The valves (fig. 5) are broad, rather flat, 

 and rounded at tip. They measure from .70 to 1 mm. in length, and are 

 attached to the stalk by a" neck " of about half that length. The stalk itself 

 is from 2.5 to 3.5 mm. long. 



The ophicephalous pedicellarioB (fig. 2) are very abundant on the ambu- 

 lacra, among the spines on the interambulacra, and on the buccal plates. A 

 few may occur on the ocular plates. The valves (figs. 7-9) are short, broad, 

 and rounded at the tip, and differ from each other in the size and form of 

 the articular loop. They measure from .50 to .75 mm. in length, and are 

 attached to stalks 4-6 times as long. There is no " neck," but the upper 

 end of the stalk is slightly expanded. 



The triphyllous pedicellariae (fig. 3) are rather scarce, but are to be found 

 on the bare parts of the interambulacra, and rarely on the genital plates. 

 The valves (fig. 6) are broad and flat, rounded at the end, with no " cover- 

 plate," but with a slight articular loop. They measure from .20 to .40 mm. 

 in length, and are attached to the stalk by a neck nearly twice as long. The 

 stalk is from 1 to 1.3 mm. in length. 



Calcareous particles appear to be quite wanting in the walls of the pedicels, 

 except for the terminal rosettes and their supporting rods in those of the 

 actinal surface, but large, flat, perforated plates occur in the gills. 



The sphoeridia are large and nearly globular. 



Arbacia spatuligera A. Ag. 



Echinus (Agarites) spatuUger Valenciennes, 1846. Voy. Venus, PI. 5, fig. 2. 

 Arbacia spatuligera A. Agassiz, 1872. Rev. Ech., pt. 1, p. 93. 



Plate 48, figs. 15-19. 



The tridentate pedicellariae are abundant in some specimens but rare in 

 others. They are very variable in form and size, but resemble those of 



