OCULAR AND GENITAL PLATES. 119 



vidual variation as regards shape, some being depressed spheroidal, others ahnost conical. 

 The largest specimen measures 145 mm. in diameter and 96 mm. in height. 



Echinus nngulosus, which is a distinctly southern species, from South Africa (100 specimens), 

 has typically, 54 %, all oculars exsert. In 30 % ocular I is insert as a progressive variant, and in 

 4% ocular V alone is insert. In 9 % oculars I, V and in 2% I, V, IV are insert. One speci- 

 men is aberrant with oculars I, IV insert. This species is distinctly more evolved as regards 

 oculars than any northern species of the genus. 



Another southern species is Echinus magcllanicus, from Patagonia and that region. A fine 

 series was studied in the collections of the Museum of Comparative Zoology and the National 

 Museum. In the veiy young, 2.5 (o 5 nun. in diameter (36 specimens), the oculars are all 

 exsert in 97%, but one specimen, the largest of the series, has ocular I reaching the periproct, 

 as in the adult typically. This immature condition of all oculars exsert is direqtly comparable 

 to the adult character of the northern Echinus esculentus. In the younger specimens studied 

 (Plate 3, fig. 14) the oculars are broadly exsert, but I is furthest in, genitals are imperforate, 

 and there is a single madreporic opening in the middle of genital 2; also the suranal plate fills 

 the periproctal area, all as in young Strongylocentrotus (text-fig. 131, p. 129). On the peri- 

 stome the primordial ambulacral plates nearly fill the area, and, as in Strongylocentrotus (Plate 

 3, fig. 11), the primordial plates 16, 116, IIIo, IV6, Va have an ambulacral perforation whereas 

 the other plates are imperforate. The largest immature specimen, which has ocular I insert, is 

 5 mm. in diameter and has four madreporic pores and two periproctal plates in addition to 

 the suranal. Adult Echinus magellanicus (164 specimens) has typically, 88 %, ocular I insert 

 (text-fig. 165, p. 149). In 5% all the oculars are exsert as arrested variants, like the young 

 and the typical character of the northern species of the genus; two of these variants were the 

 largest specimens seen of the species. In 4% oculars I, V are insert as progressive variants, 

 like the typical character of Echinus margaritaceus, and one specimen has oculars I, V, IV, 

 II insert. Two specimens are aberrant variants, one with oculars I, II insert like the typical 

 character in Gymnechinus, and one \\\{\\ V, II insert, ocular I being excluded by the fusion of 

 genitals 5, 1, as in text-fig. 146, p. 134. Echinus inagellanicus has the highest percentage of 

 ocular I onlj' insert of any sea-urchin known. In this species there are ten primordial 

 ambulaci'al plates in the peristome, as usual, but, as shown by Doderlein (1906), the 

 peristome is otherwise without plates, a very unusual character, seen, however, in some of the 

 Temnopleuridae (p. 84). 



Echinus margaritaceus, from southern South America and the Antarctic, is a most inter- 

 esting species. Sixteen specimens were studied, twelve being in the Museum of Comparative 

 Zoology, one in de Loriol's collection, and three were kindly loaned me by Dr. Mortensen under 

 the name Echinus (Sterechinus) neumayeri, but they seem referable to this species as a synonym, 

 and therefore are included with it in the tabulation. Out of the sixteen specimens, in nine, 



