120 ROBERT TRACY JACKSON ON ECHINI. 



or 56 %, oculars I, V are insert, as the typical species character. As variants, a most extraordi- 

 nary range of characters is presented for such a limited series of specimens. In one specimen 

 each, the following structural characters are shown: oculars all exsert, I insert, and V insert 

 as arrested variants, and I, V, IV insert, I, V, IV, II, and all insert as progressive variants; 

 one specimen is aberrant with I, V, II insert, a feature rather common in this family. The 

 three specimens sent me by Dr. Mortensen as above noted are two with I, V insert and one 

 with I, V, II insert. The range of normal arrested and progressive variants with the typical 

 character covered by the few specimens seen of this species, is greater than has been found in 

 any other species in the family. Koehler (1906) says of this species that in most cases two 

 oculars reach the periproct, and he shows a wide range of characters in his excellent figures. 

 In his figure 30d oculars I, II are insert, the same as I found in an aberrant E. magellanicus. 



According to de Loriol (1883), in Echinus verruculatus Liitken, from Mauritius, two oculars 

 are insert, the bivium as shown in his figure. Mortensen (1909) shows in Sterechinus anlarcli- 

 cus Koehler and S. diadema (Studer) from the Antarctic that three, four, or five ocular plates 

 may be insert. His table (1909, p. 75) shows some aberrant arrangements of oculars that I 

 have not seen in any Echini. All the evidence goes to show that the northern species of Echinus 

 have all oculars exsert, or occasionally one or two plates insert as progressive variants. On the 

 other hand, the far southern species have one, two, or more to all plates insert as a frequent 

 variant or as a typical species character (table, p. 160). 



The genus Gymnechinus is closely allied to Echinus, and* two species have been studied 

 which present extraordinary characters. Of Gymnechinus robillardi I have studied nine speci- 

 mens, seven of which are in de Loriol's collection, in the Geneva Museum. Eight of these, or 

 89%, have oculars I, II insert and V, IV, III exsert (text-fig. 179, p. 165). One specimen in 

 de Loriol's collection has ocular I only insert as an arrested variant. The essential structure of 

 the apical disc in this species is similar to that of the next. Of Gymnechinus pulchellus Dr. 

 Mortensen most generously sent me ten specimens from the Gulf of Siam and Singapore. The 

 periproct is quite eccentric (text-figs. 177, 178, p. 165), so that a line drawn through ocular III 

 and genital 5 barely cuts the periproct; the genitals are much larger on the left than on the 

 right of this line. In fourteen specimens seen, oculars I, II are insert and V, IV, III exsert in 

 all cases, and Dr. Mortensen writes me that this is always the case in this species. These are 

 the only species of Echini seen (except Strongylocentrotus gibbosus) in which an aberrant arrange- 

 ment of ocular plates is a typical species character. When two oculars reach the periproct, it is 

 typically I and V in all other Echini in over 99% of the 35,184 cases observed. For oculars 

 I, II to be insert is a relatively rare character seen in only thirty-four specimens other than 

 Gymnechinus, and all of these are in the families of the Echinidae or Strongylocentrotidae. 

 Another extraordinary character common in these species of Gymnechinus is for genital 3 or 4 

 to be exsert or excluded from the periproct (text-figs. 177-179, p. 165), as discussed under 

 consideration of genital plates. 



