170 ROBERT TRACY JACKSON ON ECHINI. 



frequently than an}^ other plate in the families of the Echinidae and Strongylocentrotidae. 

 Genital 2 is also frequently divided in .S'. drobachiensis by secondary sutures (text-figs. 191- 

 194.) Divisions of this plate were found in 120 cases in this species. Genitals 1, 4, and 5 are 

 occasionally divided by secondary sutures, as seen in 134 cases, but 2 and 3 take the lead by 

 a strong majority. Very rarely oculars are split (text-figs. 193, 194), but only ten cases were 

 noted; otherwise, genitals divided by secondary sutures are the only cases I have seen in Echini 

 of plates derived by the fission of preexisting plates. In the young of echinothuriids, genitals 

 are not split, and in Strongylocentrotus the splitting was very rare in young specimens, wliile 

 rather frequent in adults. It is evident, as in echinothuriids, that this is a character that comes 

 in with increasing age. Occasionally a line of secondary suture only goes partially through 

 a plate, demonstrating that these are true cases of splitting and not cases of accessory plates. 



In very young Echini genital pores do not exist, as shown fully by Loven (1892) and A. 

 Agassiz (1904). As seen in Strongylocentrotus drobachiensis (p. 131, text-figs. 131-134), no 

 genital pores were found in specimens up to 5 mm. in diameter, after which age they come in 

 rapidly, and only 128 in 800 were without such pores in the series 5 to 10 mm. diameter; only 

 six in 2,000 were without genital pores in the series 10 to 15 mm. diameter, and no larger specimen 

 was wholly imperforate. Occasionally, one, more rarely two, and in three cases three genitals 

 were imperforate in adults. In a fresh Toxopneusies atlaniicus examined, in which a genital 

 was imperforate, the corresponding genital gland was absent. The absence of a genital pore 

 as a variant may be compared with Holectypus (text-fig. 171, p. 149), and some spatangoids 

 (A. Agassiz, 1904) in which one or more genitals are typically imperforate. 



Typically, in post-Palaeozoic regular Echini there is a single genital pore within the con- 

 fines of each genital plate. In Goniocidaris nutrix and canaliculata (text-figs. 72-74, p. 99) 

 the ventral border of the genitals is open, doubtless resorbed so that the genital pores impinge 

 on the interambulacra. This character has been seen as a variant in Centrechinus setosus, 

 Arbacia nigra, and Strongylocentrotus drobachiensis, but is of uncommon occurrence. Mr. 

 Agassiz (1873) shows that in the clypeastroid Peronella peronii (Agassiz) the genital pores lie 

 in the interambulacra widely removed from their usual position. In an Arbacia punctulata 

 in one area the genital pore is not in its usual plate, but passes through the third interambula- 

 cral plate from the apical disc. In Strongylocentrotus the genital pores very rarely perforate 

 an interambulacral plate, and in four cases found perforated an ocular plate. While genital 

 pores are usually found in genital plates, it seems that there is no necessary correlation, and the 

 pore when developed may perforate any portion of the test. 



Genital pores are usually perfectly visible in dorsal view, are even salient features, but in 

 Salenia pattersoni they are invisible externally though plainly seen in an internal view of the same 

 specimen (Plate 4, figs. 1,2). Wliile a single genital pore to a plate is the character in Recent 

 regular Echini, supplementary pores are not rare. They are especially common in Arbacia 



