SPECIAL CHARACTERS OF GENITAL I'LATKS. 



107 



stantly in contact with the interambulacrum ventrally. In Bolhriocidaru archaica (Plate 1, 

 fig. 2) the genitals lie wholly dorsal to the oculars, and in B. pahlcn, < IM.-.i,- I , fig. 6) they do *o 

 in part. In one specimen of Slrongylocentrolus drobachiensis (Plate 5, fig. 10) genital I li.-.~ 

 dorsal to the oculars, as in Bothriocidaris archaica; but in this peculiar specimen inleramUla- 

 crum 1 is wanting (p. 42), which accounts for this exceptional genital exclusion. Otherwise, 

 as far as seen, a genital, when existent, always extends to the inleraml.ulacnun of ii> area. 



Genital plates are of secondary importance. One may be absent wit liout visibly affecting 

 the corona (spatangoids, text-figs. 174, 175, p. 149). Rarely a genital may be wanting in 

 regular Echini (p. 45), when the other portions of the test have the usual pentamerous system. 

 Such a case is seen in Eucidaris (text-fig. 185). In this specimen there are five oculars, I aixl 1 1 

 being in contact from the absence of genital 1. In interambulacrum 1 there an- two young 

 plates lying against oculars I and II; the next older plate belonging to column 2 is large and 



extends across the area, whereas the third older plate i- 

 small. Otherwise the corona is quite as usual. In two 

 specimens of Arbacia (Plate 4, figs. 11, 12) and in a 

 Strongylocentrotus (Plate 6, fig. 7), genital 4 is want- 

 ing and interambulacrum 4 is narrowed, consisting in 

 part of only a single column of plates. In Tripneustes 

 esculenlus (Plate 6, fig. 4) there are six oculars, but 

 no. additional genital, so that the extra genital may 

 theoretically be considered as wanting, and there is 

 only a single column of interambulacral plates through- 

 out the added area as far as developed. In all of these 

 specimens in the area where the genital is wanting, the 

 interambulacrum dorsally abuts against the two over- 



II 



755 



TEXT-FIG. 185. Eucidaris tribuloides (La- 

 marck). Jamaica. Diam. 34 mm. R. T. J. 



Coll., 802. Genital i wanting. Young interam- lying ocular plates in a similar manner to that of 

 bulacral plates originate against oculars I and Bothriocidaris and the posterior area in spatangoids 



II, as usual (p. 45). 



(text-figs. 162, 174, 175 p. 149). 



Genital and ocular plates are fused in a mass in clypeastroids, and two genitals may be 

 fused in spatangoids (A. Agassiz, 1904, p. 161), but in regular Echini these plates are typically 

 separate. Occasionally, however, in regular Echini two genitals or a genital and an ocular 

 are fused. In a specimen of Centrechinus setosus (text-fig. 186) genitals 2, 3 and ocular III are 

 fused into a perfectly symmetrical mass, no trace of sutures being visible. A similar fusion 

 of two genitals, or a genital and an ocular, has been seen in many cases in the Echinidae and 

 Strongylocentrotidae. In Strongylocentrotus drobachiensis genitals 2, 3 were fused in 30 

 3, 4 in 32 cases (text-fig. 195) ; 4, 5 in 68 cases (text-fig. 147, p. 134) ; and 5, 1 were fused in 19 

 cases (text-fig. 148, p. 134). Genitals 1, 2 were fused in only five cases. The fusion of genitals 



