250 
Proceedings of the Royal Society of Edinburgh. [Sess. 
(6) Strong ylocentrotus drobachiensis : 6 teeth, ambulacra, inter-ambu- 
lacra, oculars, and genitals, two of the latter adherent. Jackson, op. cit., 
p. 47. 
(7) Strongylocentrotus drobachiensis as No. 6. 
(8) Species unnamed — hexamerous specimen (no further description), 
seen by Ribaucourt and mentioned by him, loc. cit. 
Irregularia. 
(9) Galerites albogalerus : 6 ambulacra and inter-ambulacra. Meyer, 
Nov. Acta Ac. L. G. Nat. Cur., vol. xviii., 1836, p. 294, pi. xiii., figs. 6 
and 7. 
(10) Galerites sp., six-rayed. Laur., S. B. Ges. Isis, 1894, p. 6. 
(11) Pyrina ovulum : a sixth ray added at posterior, displacing periproct, 
6 oculars, 5 genitals. Seguin, Feuill. Nat., ser. 4, No. 376, 1902, p. 81, 
figs. 1 and 2. 
The descriptions of many of these specimens are insufficient to indicate 
whether or not the hexamerous arrangement prevailed in all the organs. 
As to the Pyrina ovulum recorded by Seguin, that author suggests that 
the addition of a fifth genital plate and corresponding ocular indicates a 
reversion to an earlier and more typical symmetry than that of Pyrina. 
Of those regular sea urchins which have been described with any attempt 
at detail, perfect hexamery appears to have prevailed in cases 4 and 5 of 
the above list, all the organs having been, so far as one can judge, perfectly 
formed. The origin of such cases cannot be further particularised than as 
due to spontaneous meristic variation. 
In the case of the Echinus esculentus here described the apical area 
gives a clue to the situation of the abnormality. Oculars II., III., IV., and 
Y. are similar in size, whereas the remaining two oculars resemble each 
other in being each about half the size of a normal individual. This fact, 
and the irregularity of the neighbouring genital plates, indicate that the 
additional areas were added in the right posterior segment. Further, the 
union between genitals 1 and 6 suggests that the latter is an imperfect 
double of the former, the imperfection of the duplication making necessary 
the abnormal compensatory extension of genital 5. We suggest, then, but 
with reserve, that this Echinus may represent a case of almost perfect redupli- 
cation of radii, inter-ambulacral area 1 and its flanking series of ambulacra 
being repeated in area 6 with its associated ambulacral series. If such be 
so, the present specimen comjDletes the series of already known cases of 
partial reduplication of radii (see Bateson, Materials for Study of Variation, 
