THE PENTAMEROUS SYSTEM AND \ Aid ATlo.V '.M 



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

 imperforate ocular is usual when an ambulacrum drops out i Plate 7, fig. 4) and evident!;, 

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

 from Woods Hole, Massachusetts (R. T. J. Coll., 815). It i> a -mall individual. 22 mm. in 

 diameter, and distorted. Pentamerous ventrally, the ambulacra I and IV drop out dor-ally 

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

 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 interambulacra dorsally, three oculars and Jin- 

 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-FIO. 1.- Strongylocentrotus drobachiensis (O. F. Muller). Sullivan, M:,in.-. I)i:un. mm. II. T. J. < 

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



