REPORT ON THE CRINOIDEA. 733 
B. Tue Perisomatic SKELETON. 
This name was given by Sir Wyville Thomson’ to “the basal and oral plates, the 
anal plate, the interradial plates, and any other plates or spicula which may be developed 
in the perisome of the cup or disk.” He pointed out that the plates of this system are 
“essentially variable in number and arrangement; most of the minor structural modi- 
fications throughout the group depend upon the multiplication or suppression of plates 
of this series. Even in the same species they are by no means constant,” e.g., Antedon 
rosacea. The nature of the basals and orals has been already discussed; and very little 
need be said about the anal plate. For although this forms an essential part of the cup 
of the Pentacrinoid larva of Comatula, and is of extreme importance in its paleontological 
relations, yet it disappears soon after the termination of Pentacrinoid life, undergoing 
exactly the same process of resorption as the orals have previously done. It is curious, 
however, that there should be no special anal plate in Hyocrinus, which has such large 
orals (Pl. VI. figs. 1-5), while it is also absent in the adult Rhizocrinus, and is perhaps 
never developed at all; for Sars figures a young individual only 25 mm. long in which 
the first brachials are comparatively large and form a sort of pyramid, while the second 
brachials are undeveloped, and he makes no mention whatever of an anal plate.? Whereas 
in Antedon rosacea the anal plate appears soon after the second radials (which represent 
the first brachials of Rhizocrinus) ; and it is relatively quite large by the time that the first 
brachials are developed, forming a nearly complete circle together with the first radials, 
between two of which it is intercalated. 
The interradial plates are those minute disks or granules which occur in the substance 
of the perisome uniting the rays and their subdivisions, and are sometimes difficult to 
distinguish from the lowest joints of the pinnules. They were first detected in Antedon 
milleri by J. 8. Miller,’ who figured them as forming one “intercostal” between every 
two second radials. This was probably due, as remarked by Dr. Carpenter,‘ to his having 
only employed a low magnifying power in his examination of them. Miiller’ described 
them as occurring in Pentacrinus asteria (Pl. XIIL. fig. 1), and noticed their difference from 
the plates on the ventral surface of the disk which are pierced by the water-pores 
(Pl. XVII. figs. 6, 10). They are very abundant in some species of the Comatulide and 
Pentacrinide, uniting the rays and their lowest divisions very closely together ; while in 
other types they may be wholly or entirely absent in some individuals, and more or less 
well developed in others. In fact, the same individual may have them in one or two of 
1 Phil Trans., 1865, pp. 540, 541. 
2 Crinoides vivants, p. 27. Tab. iv. fig. 95. 
3 This is the Comatula fimbriata of Miller, which occurs in Milford Haven. See his Natural History of the Crinoidea, 
Bristol, 1821, Frontispiece, fig. 2, G. 
4 Phil. Trans., 1866, p. 716. 5 Bau des Pentacrinus, loc. cit., p. 49. 
(ZOOL. CHALL, EXP,—PART Xxx11.— 1884.) h 10 
