REPOET 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 Rkizocrhius ; 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 Thaumatocrinus (PI. 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 grouji 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 Palseocrinoids 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 Eucalypto- 

 crinus, 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 Vllb. fig. 2, ai ; PL XXIV. figs. 7-9 ; 

 PL LVIII. figs. 1-3 — ai, 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 III. figs. 1, 3-15 ; PL Vb. fig. 1, A; PL Vc. fig. 2, A, figs. 3, 7, 8, 10, a; 

 PL Vila. figs. 15-23; PL Vllb. figs. 1, 5-8, A; PL Villa, figs. 5, 7, 8, A ; PL X. 

 figs. 1-4, 6-8, 17, 18 ; Pis. XII., XXL, LXTL). Chapman, who has recently attempted a 

 classification of Crinoids,^ appears to imagine that Marstipites 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 Schlliter'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 ti'ansverse 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 Palseocrinoids such as Platycrinus and some, if not all, species of Cyatho- 

 crimis; and though the arms of Platycrinus are pierced by axial canals, it is placed 

 among the " Emedullata " by Chapman. 



1 A Classification of Crinoids, ruaJ before the Eoyal Society of Canada, May 26, 1882. 



2 Op. cit; Taf iv. fig. 4. " 3 Qp. cit., p. 138. 



