SPECIAL CHARACTERS OF GENITi^L PLATES. 



171 



punclulnla (text-figs. 197, 198), in which I previously described them (Jackson, 1899, p. 130). 

 Simikir extra genital pores have been seen in Eucidaris tribuloides, Echinus affinis (text-fig. 115), 

 Strongylocentrotus franciscanus (text-fig. 199), ;S'. drohachiensis (text-fig. 193), and many other 

 species. In S. drobachiensis extra pores were seen in 186 specimens. Dr. Mortensen writes 

 me that such extra pores are connected with the genital glands. They are always sporadic 



Text-figs. 197-199.— Extra pores in genital plates. 



197. Arbacia pimclulala (Lamarck). Woods Hole, Massachusetts. Diani. '6-1 mm. 

 oculars exsert. 



198. The .same. Diam. 35 mm. R. T. J. Coll., 655. X 3. Ocular V insert (p. 115). 



199. SlrongyloceiUrolus franciscanus A. Agassiz. California. Diam. 123 mm. 11. T. J. Coll., 72(j. X 2. 

 V insert (compare text-fig. 115, p. 117). 



R. T. J. Coll., 656. X3. All 



Oculars I, 



and a]:)parently may be considered a parallel variation to, rather than a genetic connection with, 

 the Palaeozoic types where extra pores typically occur. 



In the Ordovician Bothriocidaris (Plate 1, fig. 2) genital pores are unknown; it may be 

 that they were wanting, as such pores are wanting in young Echini. More likely they existed 

 but do not show in external view, as noted in Salcnia pattersoni (p. 112). In an ancient fossil it 

 would be easy for small genital and madreporic pores to be so filled up as not to be recognizable, 

 for frequently in fossils pores cannot be seen when we know from other specimens that such 

 existed. In Lepidechinus (Plate 63, figs. 7, 8) there is one pore in each genital plate in the 

 only species of the genus in which the apical disc is known. 



In other Palaeozoic Echini genital plates typically have more than one pore to a plate. 

 There may be two or three, as in Lepidesthes (Plate 68, fig. 5), or there may be three to five 

 in a plate, as in Palaeechinus (Plate 29, fig. 6; Plate 30, fig. 4), Lovenechinus (Plate 41, fig. 3), 

 and Melonechinus (Plate 56, fig. 6). Instead of a few pores there may be numerous genital 

 pores to a plate, even as many as ten or eleven, as shown in Lepidocentrus (Plate 21, fig. 5), 

 Pholidechinus (Plate 28, fig. 10), and Perischocidaris (Plate 67, fig. 3). It is po.ssible that in 

 types where fine madreporic pores are unknown, some of the larger pores served as madreporic 



