166 ROBERT TRACY JACKSON ON ECHINI. 



250,000 genital plates have been examined, and the principal striking variations noted. In 

 the 33,000 specimens of Strongylocentrotus drobachiensis studied, detailed lists of the variations 

 of genital plates were kept as a basis for determining the numerical frequency of occurrence. 



Under ocular plates it is shown that there is great diversity as to whether these meet the 

 periproct or are shut out from it. On the other hand, genital plates show great uniformity 

 in that all the plates meet the periproct. No exception to this is known in fossil regular Echini; 

 in Recent Echini, however, some exceptions have been found as individual variations. 



Dr. Mortensen (1904, p. 114) says of Gymnechinus pulchellus that sometimes one of the 

 genitals, mostly 3, is excluded from the periproct. He writes me that in about thirty percent of 

 the specimens genital 3 and more rarely 4 is exsert. In ten selected specimens of G. pulchellus 

 that Dr. Mortensen generously sent me, four have genital 3 exsert (text-fig. 177), two have 

 genital 4 exsert (text-fig. 178), and four have all genital plates reaching the periproct. In a 

 specimen of Gymnechinus robillardi (text-fig. 179) genital 3 is exsert. De Loriol (1883) figures 

 the same species with genital 4 exsert. In his collection I saw two specimens with genital 4 

 exsert, as in text-fig. 178. It is striking that this very exceptional character is common in two 

 related species. Specimens with exsert genitals have been found in a few other species. One 

 specimen in 139 Microcyphus maculaius (this is in de Loriol's collection in Geneva), one in 1,043 

 Toxopneustes varicgatus (text-fig. 184), and two in 2,643 Toxopneusies atlanticus have genital 

 3 exsert. In 33,000 Strong ylocentrotus drobachiensis 37 specimens have genital 3 exsert (text- 

 fig. 180). In all the cases seen of genital 3 exsert in the several species, this plate was similarly 

 excluded by the contact of genitals 2 and 4 as figured. In S. drobachiensis eight specimens 

 have genital 4 exsert. This, however, is not excluded by the same plates in all cases. In a very 

 young individual (text-fig. 181) genital 4 is excluded by the contact of genitals 3 and 5, as in 

 Gymnechinus pulchellus (text-fig. 178). Another method of exclusion of genital 4 is by 

 the contact of genital 5 and ocular IV (text-fig. 182) ; and in a third method, which may be 

 considered more normal for the species, genital 4 is excluded by the contact of ocular V and 

 genital 3 (text-fig. 183). In the Museum of Comparative Zoology, a Microcyphus maculatus 

 from Mauritius has genital 4 excluded by the contact of ocular V and genital 3, as in text-fig. 

 183; also a Heterocentrotus trigonarius from the same locality has genital 3 excluded by the 

 contact of genitals 2 and 4, as in text-fig. 184; and a Microcyphus annulatus Mortensen, from 

 Bass Straits, Australia, has genital 1 excluded from the periproct by the contact of genitals 5 

 and 2. The oculars of this specimen are all exsert, as usual in the genus. 



In all, 61 cases have been seen in 50,000 specimens with a genital excluded from the peri- 

 proct, 47 of the cases being of genital 3, thirteen of genital 4, and one case of genital 1. Why 

 genital 3 or 4 should be excluded and almost no others is not obvious, Init it shows how definite 

 even a very rare variation can be. 



While genitals are rarely excluded from the periproct as variants, they are still more con- 



