REPORT ON THE CRINOIDEA. 151 
case in Belemnocrinus, and has been thought to afford the only point of difference between it 
and the cup of the recent Rhizocrinus ; though the structure of the stem is entirely different 
in the two types. More commonly, however, there is a considerable development of plates 
in the anal interradius, and the corresponding basal plate, together with the two radials 
that rest upon it, differ from their fellows in size and shape. There is, however, one 
Neocrinoid, the aberrant Thawmatocrinus (Pl. LVI. figs. 1-5), which has the symmetry 
of its calyx disturbed by the presence in the anal interradius of a jointed appendage, 
somewhat similar to that of Reteocrinus from the Trenton group of America, and of 
Taxocrinus, Onychocrinus, &c. But there is no other modification of the plates, the five 
basals, the lowest interradials, and the radials being all equal and similar to their fellows. 
Even in Palzeoerinoids which have the radials closely united all round, and the general 
contour of the calyx perfectly regular, a want of symmetry is indicated by the inequality 
in the numbers of basal and radial plates. This is the case, for example, in Hucalypto- 
erinus, Which has five radials but only four basals. 
In all Neocrinoids (with the possible exception of the doubtful Comaster) the basals 
are pierced by interradial canals or grooves, which lodge the cords proceeding from the 
angles of the chambered organ. These cords fork and pass on into the radials, where they 
occupy the more or less double axial canals (Pl. VIIb. fig. 2, av; Pl. XXIV. figs. 7-9; 
Pl. LVIII. figs. 1-3—az, ar). These axial canals open on the distal faces of the radials in 
the centre of the transverse articular ridge, and are thence continued into the rays and 
arms (PL, WT. figs..1, 3-15; Pl. Vb. fig. 1,,A; Pl. Ve. fig, 2, A, figs. 3, 7, 8; .10,a@; 
PEVile. fests 235, Pl Villbs figs, 1575-8). A\ 5) PL Villas fies.) 5;,7,. 85 Ave Ble xe 
figs. 1-4, 6-8, 17, 18; Pls. XII., XXI., LXTI.). Chapman, who has recently attempted a 
classification of Crinoids,’ appears to imagine that Marswpites and Uintacrinus have 
imperforate radials ; for he places them both in his Division 1, ‘“ Emedullata,” which he 
characterises as follows, “ Calyx and arm-plates without internal canals.” I know nothing 
about the calyx-plates of Uintacrinus; but the central canal of the third brachial is 
shown in Schliiter’s figure ;” while the perforation of the radials of Marsupites was 
mentioned by Miller,’ and has been figured by many later writers. 
The presence of a transverse ridge on the articular faces of the radials indicates that 
they were united to the second radials by muscles and ligaments, just as the successive 
arm-joints are. This is invariably the case in all Neocrinoids except Guettardicrinus, 
which has the plates suturally united, and possibly also in Uintacrinus. I have pointed 
out already that there was a fully developed muscular joint between the two lowest 
radials of many Palzocrinoids such as Platycrinus and some, if not all, species of Cyatho- 
crinus; and though the arms of Platycrinus are pierced by axial canals, it is placed 
among the “ Emedullata” by Chapman. | 
1 A Classification of Crinoids, read before the Royal Society of Canada, May 26, 1882. 
2 Op. cit., Taf. iv. fig. 4. 3 Op. cit., p. 138. 
