204 Transactions. — Zoology. 



When a good series of this species— such as I have figured 

 on Plates X., XI., and XII.— is carefully examined the most 

 remarkable and peculiarly-interesting feature is seen to be, not 

 so much the irregularity of the rays — which is, however, suf- 

 ficiently striking— but, if I may use the expression, the 

 regularity of the irregularity. A large number of specimens, 

 especially the smaller ones, are seen to have the three adjacent 

 rays on one side of the disc smaller and less developed than 

 those on the other side, and the part of the disc to which the 

 group of smaller rays is attached is less developed than the 

 other part, and separated from it by a distinct depression, 

 showing clearly that the specimens have undergone transverse 

 division._ The normal fully-grown forms have each six rays, 

 four madreporiform plates, an elliptical disc, and a sulcus run- 

 ning across the narrower part of the disc. I have figured on 

 Plates X., XL, and XII. the whole series collected by me during 

 the summer of 1893-91, and, although a little dredging in deep 

 water would perhaps produce some larger specimens, I have 

 no doubt that they are fairly representative of the species. 

 The madreporiform plates of each specimen are shown, and 

 the sulcus, which marks the place of division, is indicated by 

 a broken line. The figures are reduced to half the natural 

 size. Of the sixty-seven specimens figured, fifty-five have six 

 rays each ; fifty-three of these show clearly that they have 

 suffered division by the arrangement of the rays in two groups, 

 the position of the madreporiform plates, and the sulcus 

 across the disc. Of the rest, four specimens— PI. X., figs. 5, 

 10, 23, and 28 — have seven rays each. These are evidently 

 abnormal forms, which have either divided unequally (figs. 10 

 and 28), or four rays have grown in the place of three (figs. 5 

 and 22). Four specimens— PI. X., tigs. 17 and 31 ; PI. XL, fig. 3; 

 and PL XII, fig. 7 — have only five rays each, one of which — 

 PI. XL, fig. 3 — was probably always a five-rayed individual, and 

 shows no sign of division ; but the other three show clearly 

 that they have undergone division : they were probably six- 

 rayed forms when young, but after division only two rays were 

 developed in the place of the three. Only one specimen of the 

 whole series, which is a five-rayed individual, has a madre- 

 poriform plate adjacent to the sulcus which marks the place of 

 division (PI. XII., fig. 7), but evidently the sixth ray, had it 

 been developed, would have intervened here. Three specimens 

 — PI. X., fig. 25, and PI. XII., figs. 13 and 11— have only three 

 rays each. These had evidently divided shortly before they 

 were collected, and the other rays had not begun to grow. 

 Two of these— PI. XII., figs. 13 and 11— are especially interest- 

 ing. I found them close together. The rays of both are 

 equally developed, and are the same size, shape, and depth of 

 ■colour. They are, I have no doubt, the parts of a six-rayed 



