MORPHOLOGICAL PART. 127 



of the inferradial, which retained its place between the upper sloping faces 

 of two basals, only changing its outlines to conform to the shape of, and fit 

 in between, adjoining plates. In the Carboniferous, in which the ventral sac 

 attained its greatest dimensions, and the lower faces of the costals frequently 

 fill up the whole width of the radials, leaving no space upon their distal faces 

 for an attachment of the tube, the superradial, R, was shifted to the right, 

 into a position almost directly above the right postero-lateral basal, and 

 obliquely against the inferradial R' {Foteriocmms, Fig. 2). Finally toward 

 the close of the Carboniferous, when the tube was reduced to its primitive 

 insignificance, the anal plates gradually disappeared, and the ^ye radials re- 

 sumed their normal position as in Urisocrinus, Stemmatocrinus, and Encrinus. 

 This is entirely different from the interpretation of Mr. Bather, who 

 reaches the conclusion that the anal plate x was primitively derived from a 

 brachial, and that in geological time it passed down from above into the 

 dorsal cup. He regards {op. ciL, p. 329) " those forms as primitive in which 

 the radianal is more of a radial and less of an anal, in which it is not in an 

 asymmetrical position but corresponds to the other lower brachial plates. 

 Such forms are locrinus, Ileterocrinus, Ectenocrinus, Anomalocrinus^ and Mero- 

 erinus. Now in all these forms x is supported by R, and does not touch i?'. 

 Obviously then x is not derived from R', but originates above i?, and on its 

 left side. By parity of reasonmg we assume the next stage to be repre- 

 sented in such forms rs ITyhocmius (1), Ottaioacrinus, Bendrocrinus, and Homo- 

 crinus, since in them R is rather more asymmetrical. In these x has 

 passed down from above R, and now^ rests with its lower half between the 

 right and left posterior radials, being supported partly by R' and partly by 

 the basal. Carabocrinus, Botryocrinus, and similar forms are, as all acknowl- 

 edge, the next stages in the shifting of the radianal ; in these x has sunk 

 still lower into the dorsal cup, and is now entirely in a line with the radials. 

 ... In Parisocrinus and Euspirocrinus among pinnuless forms, and in the 

 Poteriocrinites, another change has taken place ; the radianal has passed 

 through a revolution of 90°, and the lowest plate of the ventral sac {t) has 

 sunk down between R and ^." 



Before inquiring into the validity of this argument, it will be well to ascer- 

 tain whether the plates of the different genera which Bather calls x are 

 structurally the same thing, for an error in this respect would be fatal to the 

 whole theory. The question is, is his plate x in locrinus and Merocrinus, 

 which rests upon the one marked by him R or (7, and that resting upon R! 



