40 THE VOYAGE OF H.M.S. CHALLENGER. 
Wyville made no further reference to them except to say that they sometimes remained 
permanently in the adult Antedon, usually in groups of three or five. These last, 
however, like those already noticed in Pentacrinus (Pl. XIII. fig. 1), are merely periso- 
matic plates developed in the tissue uniting the second radials. Good figures of them 
were given by Dr. Carpenter ;* and it is by no means certain that they are a further 
development of the primitively single plates which appear between the orals and basals, 
for the direct continuity of the two structures has never been definitely traced. 
I am inclined to believe that where they do appear in ordinary Comatule these 
primary calyx-interradials eventually undergo resorption like the orals and the anal 
plate. But they are permanent in Thawmatocrinus (Pl. LVI. figs 1-4), as are also 
the orals (fig. 5). This remarkable genus has five calyx-interradials which rest on 
basals and separate the radials just as in certain Ophiurids and in some types of the 
Palaeozoic Rhodocrinidee. It is much to be regretted that this extraordinarily interesting 
form is only represented by a single individual. For the study of the distribution of 
the axial cords within the calyx would have been of some importance. 
If Thaumatocrinus resembles the ordinary Comatule and Pentacrinidz, the circular 
commissure (Pl. XXIV. fig. 9, c.co., 7.co.) is formed by both interradial and intra- 
radial commissures which connect the paired branches of the five primary interradial 
cords (Pl. XXIV. figs. 7-9; Pl. LVIIL figs. 1-3—av); and the interradial commissures 
must traverse, or at any rate, lie upon the inner surfaces of the interradial plates. But 
the general embryonic characters of Thawmatocrinus lead me to think that the arrange- 
ment of its axial cords must be more like that which occurs in Bathycrinus. In this 
genus the primary cords do not fork within the basal ring, but pass upwards through it 
and enter more or less complete canals which are formed by the apposition of two 
grooves, one on each of the contiguous lateral faces of adjacent radials (Pl. VII. fig. 6a). 
When they reach about half the height of the radials they fork, and the resulting 
branches themselves form the interradial commissures, entering the radials by the 
apertures in their lateral faces. 
I cannot help suspecting that the same condition may occur in Thawmatocrinus, 1.e., 
that the primary interradial cords run upwards through the basals into the interradials, 
and there fork, one branch entering each of the two radials which are separated by the 
interradial lodging the primary cord. This simple condition would correspond very 
well with the general embryological characters of the type, as revealed in other ways. 
But owing to the want of material there is unfortunately but little chance of the above 
hypothesis being verified or disproved. 
While resembling the Rhodocrinites in having five large plates separating the radials, 
Thaumatocrinus differs from most Palocrinoids, with the exception of the Platycrinide, 
in the absence of any higher series of interradial plates. Except on the anal side these 
1 Phil. Trans., 1866, pl. xxxili. fig. 7 A, B. 
