CRINOID FROM THE SOUTHERN SEA. 
927 
in Reteocrinus and in Xenocrinus (Plate 71, fig. 8) the isolated radials are quite 
narrow, and the interradial spaces separating them are sometimes twice their breadth. 
How did the circular commissure traverse these spaces ? Whether there were canals 
in the radials or not, the interradial portions of the commissures must have been 
freely exposed to the body-cavity at the bottom of the calyx, for it is not likely either 
that the commissure was absent, or that it was situated within the ring of basal 
plates. In this respect also, therefore, Reteocrinus presents an embryonic feature, 
for in the earlier stages of Pentacrinoid life the axial cords simply lie on the ventral 
surface of the radials and brachials, without having any channels hollowed out in 
these plates for their reception, as is subsequently the case ; and whatever was the 
case with the radials and basals of Reteocrinus, it is improbable that the irregular 
interradial plates which were crossed by the cords were in any way grooved for 
their reception/'' These may fairly be regarded as corresponding to the numerous 
irregular plates which occur upon the disc and between the rays of many Neocrinoids. 
Those- of Reteocrinus, however, do not stop at the level of the second radials, but 
extend right down to meet the basals. Sometimes there appear to be only one 
or two between every two radials, e.g., R. gracilis, but in other cases the number 
seems to be larger, and the plates can hardly be regarded as the complete morpho¬ 
logical equivalents of the larger and more regular single interradials which occur in 
the Rhodocrinidce. 
The interradial portions of the circular commissure must have passed over the 
ventral surface of these large plates, if not actually piercing them. The same must 
be the case in Thcmmatocrinus, and unless its radials are different from those of all 
other Neocrinoids, the axial cords must be lodged in canals, which is probably also 
true for the interradials. In any case, however, the relation of these plates to the 
axial cords shows that they belong, like the radials and basals, to the radial system, 
rather than to the perisomatic. It is not easy to make out their homologies in other 
Echinoderms, but they are perhaps represented in the disc of an Ophiurid by the 
proximal row of intermediate plates, while the interi'adials generally correspond to the 
distal rows. 
The morphological differences involved in the separation or lateral union of the 
* Muller, and more recently Zittel, have considered the presence or absence of canals within the 
calyx-plates as affording an important character which distinguishes the Palaeozoic from the younger 
Crinoids. There are many Palseocrinoids, however, in which these canals are present, e.g., Allagecrinus, 
Platycrinus, and all forms with true articular facets on the distal faces of the radials. Platycrinus has, 
nevertheless, been placed by Professor Chapman in his division Pmedullata, the calyx and arm-plates of 
which are “without internal canals” (See “A Classification of Crinoids,” read before the Royal Society 
of Canada, May 26, 1882). Strangely enough, Marsupites is placed in the same division, although any 
specimen with a good articular surface on the radials shows the opening of the central canal as dis¬ 
tinctly as possible, and the canal actually pierces the substance of the plate, not ending abruptly on its 
ventral surface, as in the radials of Gupressocrinus. 
6 c 2 
