400 Variation in " Ophiocoma Nigra" 



almost invariably regulär in shape, either circular or pentagonal, but it did not 

 follow that a circular disc had a circular colour-patch and a pentagonal disc 

 a pentagona! one. 



The shape, but not the relative size, of the centi'al coloured part of the disc is 

 shown in Figures 4, 5, and 6. Colunin A gives the nuniber of speciineus which 

 were cntiroly of one colour, that colour beiiig as stated, neariy ahvays dark brown 

 or black. (All were viewed with aboral surface upwards.) There was very little 

 colour-variation in the rays, although generally they were not so dark as the disc, 

 but in 2"8 per cent. of 7*, the rays were ringed with grey or white at every fifth set 

 of plates for about half tlie length of the ray, beginning at the tip. In colunin B 

 are given the nuinbcrs n\' tlinsc whose disc had a sniali coloured patch, often a 

 niere speck, neariy ahvays cirevdar (as in Fig. 4) and never oecupying niore than 

 one-fourth of the whole disc. Colunin C gives the nunibers in whieh tlie coloured 

 part was greater tiian oneipiartor, but less than one-half of the disc. 16'9 per cent. 

 had this marking, which was neariy aiways pentagonal (as shown in Fig. 5), 

 although a few had the patch circular. Column D gives the numbers with a 

 coloured part greater than one-half, but less than three-fourths of the area of the 

 disc. This was aiways pentagonal (Fig. 5), and was found in 15 per cent. of 

 the speciniens. The numbers in coiunins E and F are those in which the size 

 of the coloured part of the disc was the same as that in coluinns C and D 

 respectively, while the shape was the same as that represented in Fig. G. It is 

 noted in column G that in lO'ö per cent. of the speciniens, at least three-fourths — 

 sometinies aimost the whoie — of the disc was sandy-coioured, although the rays 

 still retained their characteristic brnwii appearance. The shape of the colour- 

 niarking in this case was usuaily of the pentagonal form shown in Fig. 6, the part 

 of Ihe disc at the point where each ray leaves the disc beiug usuaily of the colour 

 of the ray. The conclusion then is that as regards the disc and its colour- 

 niarkings, 



1. 24'1 per cent. were entireiy of one colour. 



2. 11<S per cent. had a smail circular (Fig. 4) coloured patch. 



3. 31J) per cent. had a medium pentagonal (Fig. .5) coloured patch. 



4. 217 per cent. had a medium pentagonal (Fig. 6) coloured patch. 

 h. lü'5 per cent. had a large pentagonal (Fig. G) coloured patch. 



It is thus Seen (1) that in over 75 per cent. of the speciniens the disc had a 

 coloured patch, (2) that the shape of this coloured patch seemed to vary with its 

 size (being circular when smail and pentagonal when largo), and (3) that the shape 

 of the patch, likc the shape of the disc itsclf, was in general pentagonal. 



3. Correlation between Arm-length and Disc-breadth. 



To measure the widtli of the disc I u.sed a pair of tinely pointed dividers, care 

 being taken in the Operation that the points touched, but did not press against the 

 opposite sides of the disc. When the disc was pentagonal in shape, one point of 



