168 



ROBERT TRACY JACKSON ON ECHINI. 



was seen in other species also, as Tripneustes esculentus (text-fig. 196). In 103 cases of S. 

 drobachiensis an ocular was fused with a genital (text-fig. 143, p. 134), any ocular of the five 

 fusing with one or rarely two of the associated genitals. As far as the fusion of genitals, or 

 genitals with an ocular goes, it may be considered a parallel variation to the typical condition 

 in certain irregular Echini, where such fusion normally occurs. 



While fusion of genitals is rare, splitting on lines of solution to make divided plates is more 

 common. This splitting of genitals by secondary sutures is a typical character in the Echino- 

 thuriidae only (Asthenosoma, text-fig. 101, p. 109, and Phormosoma, text-fig. 170, p. 149). 

 Such splitting is shown fully by Mr. Agassiz (1904) in many of this family. The splitting is 



186 



TEXT-FIGS. 186-189. Fusion, or splitting of genital plates in Cenlrechinus setosus (Leske). 



186. Jamaica. Diam. 49 mm. R. T. J. Coll., 803. X 2.8. Genitals 2, 3 and ocular III are fused in a mass; oculars 

 I, V, IV, II insert, bilaterally symmetrical through III, 5. 



187. Bahamas. Diam. 85 mm. R. T. J. Coll., 714. X 2.8. Genitals split by secondary sutures. 



188. Bahamas. Diam. 85 mm. R. T. J. Coll., 689. X 2.8. Madreporic pores in genitals 2, 3, and ocular III. 



189. Jamaica. Diam. 71 mm. R. T. J. Coll., 772. X 2.8. Madreporite split by secondary sutures. 



especially marked on the ventral border of genital plates or may pass directly through the plate. 

 I have seen no splitting of genitals in fossil Echini, but it is rather common as a variant in Recent 

 species. Such secondary division of genitals is seen in Centrechinus (text-figs. 187, 189). 



In Strongylocentrotus drobachiensis many cases of split genitals were found. In 275 cases 

 genital 3 is split by a transverse suture parallel to the base, as in text-fig. 190. This seems to 

 be a family peculiarity, as it was observed in several species of the Echinidae as in Tripneustes 

 esculentus (Plate 6, fig. 4), and in the Strongylocentrotidae. Instead of splitting by one trans- 

 verse suture, as in text-fig. 190, genital 3 may be divided by one or more sutures in other 

 planes, as in text-fig. 190a and 190b; such were noted in 90 cases. Genital 3 is split much more 



