Studies of Jamaica Echini. 



151 



at this stage dropped into the phase of an arrested variant, though it was 

 the dominant character in the series 70 to 90 mm. in diameter and is the 

 adult character in a series from the Bahamas. Forty per cent have oculars 

 I, V, IV, II insert. This is the first series in which this character is a domi- 

 nant feature at Montego Bay, and for all earlier stages in this locality and 

 for all other localities known it is a progressive character. Two per cent 

 have all oculars insert as a progressive variant (fig. 14). It is somewhat 

 striking that with four oculars insert as a dominant character, there should 

 not be more specimens with all oculars insert. In this series, 25 specimens, 

 13 per cent, have an aberrant arrangement of ocular plates. Of these 

 aberrants, 22 have oculars I, V, II insert; two have I, V, IV, III insert (fig. 

 16), a very rare variant in the Echinidae when four plates reach the peri- 

 proct, but although rare here, it is a common character in the Cidaridae 

 and a not rare variant in Centrechinus setosus. One aberrant specimen of 



n IV 



15 



16 



17 



Figs. 15-17. — Aberrant ocular plate arrangement in Tripnettsles esculentus (Leske), Montego 

 Bay, Jamaica. All figures nearly X 2. 



Fig. is. — Diameter lis mm. Oculars I, IV, II insert, a common aberrant variant, a right-handed equiva- 

 lent of the normal character I, V, IV insert. 



Fig. 16. — Diameter 106 mm. Oculars I, V, IV, III insert, a rare aberrant variant, but the usual one when 

 four oculars but not I, V, IV, II are insert. 



Fig. 17. — Diameter 102 mm. Oculars I, V, II, III insert, a unique aberrant variant, a right-handed equiva- 

 lent of I, V, IV, III insert. 



Tripneustes has oculars I, V, II, III insert (fig. 17). This, which is a 

 right-handed equivalent of I, V, IV, III (just as I, V, II is a right-handed 

 equivalent of I, V, IV) is a unique variant for Echini as far as my experi- 

 ence goes. In the Centrechinoida, as tabulated in my preceding memoir 

 and in the present paper, 619 specimens are recorded, in which four plates 

 reach the periproct. Of these, in 554 specimens (89.5 per cent) it is oculars 

 I, V, IV, II the bivium and posterior pair of the trivium, in 64 specimens 

 (10.3 per cent) it is the aberrant arrangement I, V, IV, III, and in the one 

 specimen cited (0.16 per cent) it is I, V, II, III. (Compare figs, 13, 16, 17.) 

 The series containing the largest specimens, no to 132 mm. diameter, 

 82 specimens, has 13 per cent with oculars I, V insert as arrested variants 

 (fig, 11); 24 per cent with I, V, IV insert as arrested variants (fig. 12); and 

 43 per cent with I , V, IV, 1 1 insert as the local character of the species (fig. 

 13). Five per cent have all oculars insert as a progressive variation, and 15 

 per cent, 12 specimens, are aberrant in ocular arrangement. Of these 



