THE PENTAMEROUS SYSTEM AND VARIATION. 



37 



dorsally. There are five oculars and genitals, but oculars III and IV are imperforate. An 

 imperforate ocular is usual when an ambulacrum drops out (Plate 7, fig. 4) and evidently is a 

 correlated structure. The same structure is shown also in a specimen of Arbacia pundulata 

 from Woods Hole, Massachusetts (R. T. J. Coll., 815). It is a small individual, 22 mm. in 

 diameter, and distorted. Pentamerous ventrally, the ambulacra I and IV drop out dorsally 

 so that interambulacra 5, 1, and 3, 4 are in contact dorsally. Five oculars and genitals 

 are in place dorsally but somewhat distorted. There are as an independent variation eight 

 plates in the periproct of this specimen. 



3. Pentamerous ventrally, three ambulacra and interamhulacra dorsally, three oculars and five 

 genitals in place, two oculars out of place. — An ocular or a genital may be wanting, but when 

 present, they are almost universally in place in the apical disc. No case of a disjunct genital 



Text-fig. 1.— Slrongijlocentroliis drobachiemis (O. F. Miiller). Smllivan, Maine. Diam. 42 mm, R. T. J. Coll., 

 710. X 3.5. Oculars I and IV with associated coronal plates displaced. 



