ARBAOIA STELLATA. 71 



Arbacia stellata Gray. 



» 



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

 Arbacia atellata Gray, 1835. Pioc. Zool. Soc. Londou, p. 38. 



Plate 48, figs. £0, 21. 



The tridentate pedicellariae appear to be wanting, for none were found 

 in the ten specimens examined. 



The ophicephalous pedicellariae, which resemble those of ILrnia, vary 

 greatly in abundance in different specimens. They also vary considerably 

 in size, the length of the head ranging from .35 to .65 mm. The heads of 

 the stalks are often somewhat constricted as in pmctulata. 



The triphyllous pedicellarijB are scarce and small, the valves (fig. 20) 

 measuring only .25 mm. in length. The latter are remarkably narrow, more 

 slender even than those oi jmnchdata. 



The calcareous particles in the pedicels seem to be very characteristic, for 

 in all the specimens examined large, swollen, knobbed, and perforated plates 

 (fig. 21) were found. These are sometimes numerous and sometimes few, 

 and it is possible that they are sometimes wanting. The plates in the gills 

 are flat and smooth, as in Diifrcsnii. 



The sphoeridia are like those oi punctulaia. 



Arbacia punctulata Gray. 



Echinus punctulatua Lamarck, 1816. Anim. s. Vert., Ill, p. 47. 

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



Plates 47, figs. 17-19 ; 48, figs. 1-9. 



The tridentate pedicellariae are abundant in some southern specimens, 

 but are usually infrequent, and in northern specimens are often entirely 

 wanting. They are to be found, when present, chiefly on the actinal surface. 

 They are very variable in size, the valves ranging from .20 to 1.30 mm., 

 and also show considerable diversity of form. They intergrade with the 

 triphyllous pedicellariae very clearly. The valves are either broad and regu- 

 larly tapering to a blunt point (PI. 48, fig. 2), or narrower and distinctly 

 constricted near the middle (PI. 48, fig. 1). 



The ophicephalous pedicellariae are common, but chiefly actinally and par- 

 ticularly on the buccal plates; they are abundant abactinally only in occa- 



