2fi2 PROCEEDINGS OF THE ACADEMY OF [1885. 



crinoid. We refer to the development of the anal plate from the 

 so-called azygous piece. That a plate which takes such an im- 

 portant part in the phylogeny of this group should be altogether 

 unrepresented in the young Neocrinoid, seems to us somewhat 

 doubtful, the more so as the Neocrinoidea are in all probability 

 the pala?ontological successors, if not the linear descendants, of 

 the Inadunata. Possibly the undivided azygous plate, as repre- 

 sented in Baerocrinu8, has been overlooked in the early larva, 

 and this would not be surprising, as the plate occupies the posi- 

 tion, and has very nearly the form of an ordinary first radial. 



In our chapter on the radials we have already alluded to the 

 azygous piece, and expressed our conviction that its gradual 

 resorption gave origin, not only to the right posterior radial, but 

 also to the anal plate. We have shown that in Haplocrinus, a 

 close ally of Baerocrinus, the fifth radial is somewhat rudimen- 

 tarily represented by a small trigonal piece occupying the right 

 upper corner ; that this genus, as yet, had no anal plate, the left 

 corner of the azygous piece being still intact ; that in Hybocrinus 

 the left side of the plate was taken up b}' a small anal, and the 

 azygous piece proportionally diminished in size ; that in the 

 succeeding stages, which arc typified by Iocrinus, Dendrocrinus, 

 Homocrinus, the size of the anal plate gradually increased as the 

 azygous piece diminished; and that at last in Cyatfiocrinus 

 the latter plate was entirely removed, and the anal plate took the 

 position of that in the larva of Antedon. These modifications 

 were introduced, as a general rule, in geological succession, but 

 not always uniformly, for in some groups the development went 

 on more rapidly than in others. Such a rapid development took 

 place in Gyathocrinus, which existed already in the Silurian, 

 although attaining its maximum representation in the Carbon- 

 iferous ; while in most of the Poteriocrinida:, which eminently 

 belong to the Carboniferous, the very opposite is observed. The 

 most remarkable deviation in this respect is shown by the sym- 

 metrical Silurian genus Oodiacrinus, which apparently has 

 neither azygous nor ana! plate. 



The final resorption of both plates is best shown in the 

 Poteriocrinidse. 1 u Poteriocrinus, Eupachycrin us and Zeacrin us, 

 the azygous plate is comparatively well developed, but com- 

 pletely pushed OUt of the radial position which it had previously 

 occupied, in these genera the anal plate is -mall, and the first 



