1913-14.] Abnormal Echinoids in the Royal Scottish Museum. 249 
being two individuals on the double plate. The pore on genital plate 3 
is doubled, the apertures being incompletely separated. 
There are six oculars. Those opposite ambulacral areas II., III., IV., 
and V. are normal in shape and position. Ocular VI. lies in a notch in 
genital 5, by which plate alone it is subtended ; and ocular I. lies in a 
similar notch opposite the middle of the double genital plate. Each of 
these is about half the usual size. 
The anus lies nearest to the right-hand corner of genital 5. 
The removal of Aristotle’s lantern disclosed the fact that, in the soft 
parts also, hexamery prevailed, for the reproductive organs were developed 
in six equal inter-radial rays. The alimentary canal, although longer than 
usual, followed the normal course even in detail, for the intestine doubled 
upon itself in inter-ambulacral area 2, and its last point of attachment was 
in inter-ambulacral area 3, as in normal specimens. 
Clear cases of spontaneous variation in the major symmetries of sea 
urchins are very rare. Of the most likely of such cases — those in which 
complete rays are added — few have been recorded, and still fewer have been 
satisfactorily described. As Bateson in his Materials for the Study of Varia- 
tion, 1894, mentions only two cases of “ total variation to a six-rayed form,” 
and as Jackson’s list (op. cih, 1912, p. 46) omits several recorded cases, we 
give the following short summary of all the examples of “total” hexamery we 
have been able to discover, with the view of simplifying future researches : — 
Recorded Cases of Total Hexamery. 
Regularia. 
(1) Amblypneustes sp. : hexamerous specimen (no further description). 
Haacke, Zool. Anz., 1885, p. 506. 
(2) Echinus eseulentus : 6 teeth, pairs of buccal tube feet, ambulacral 
and inter-ambulacral zones, ocular and genital pores, ocular and genital 
plates, two of the latter adherent — described in present paper. 
(3) Tripneustes eseulentus : 6 teeth, ambulacra and inter-ambulacra, 
oculars and genitals, two of the latter adherent. Jackson, Mem. Boston 
Soc. Nat. Hist., vol. vii., 1912, p. 46. 
(4) Toxopneustes lividus : 6 teeth, pairs buccal tube feet, ambulacra, 
genitals, and oculars. Ribaucourt, Gomptes rendus Ac. Sci., vol. cxlvi., 1908, 
p. 92. 
(5) Paracentrotus (Strong ylocentrotus) lividus (artificially reared) : 
6 teeth, terminal tentacles, and ambulacra. Delage, Gomptes rendus Ac. 
Sci., vol. cxlv., 1907, p. 546. 
