BevieiDS — Wachsmufh ^ Springer'' s Ponograph on Crinoids. 427 



I believe the following to be a true summary of tlie facts con- 

 cerniug the structure of the base : — 



Fig. 4. — Bases and their modifications. 

 i-vi and ix, monocyclic ; vii and viii, dicyclic, 

 i-iv, pentagonal, unaffected by anal. 

 V, vi, ix, hexagonal, affected by anal. 

 In all the anal side is uppermost ; the imaginary additional piece is marked +• 

 i, 5 BB ; ii, 4 BB ; iii, 3 BB, Crinoid type ; 

 iv, 3 BB, Blastoid type ; v, 4 BB ; vi, 3 BB ; 

 vii, 3 IBB, as usual in Dicyclica Inadunata ; 

 viii, 3 IBB, as usual in Flexibilia Impinnata; 

 ix, 2 BB. 



The first stage is the fusion of one pair, producing one large and 

 three small plates (Fig. 4, ii). This is almost entirely restricted to 

 Monocyclic genera, where the plates that fuse are r. and 1. ant. BB. 

 Next comes the fusion of two pair, producing one small and two 

 large plates (Fig. 4, iii). This occurs in both Monocyclica and 

 Dicyclica. In the former the small plate is 1. ant. B, or rarely 

 1. post. B, whereas in Eublastoidea it is r. ant. B (Fig. 4, iv). 

 In Dicyclica 3 IBB have been observed only among Inadunata and 

 Flexibilia; in the former group the small plate is often, but not 

 always, ant. IB (Fig. 4, vii) ; in the Palasozoic representatives of the 

 latter it is [npud W. & Sp.) always r. post. IB (Fig. 4, viii), but this 

 is not the case in Antedon. A bipartite base is formed only in a few 

 Monocyclica ; the two plates lie on the r. and ]. sides of the sup. 

 (Fig. 4, ix). Finally, all plates of the proximal circlet may fuse into 

 a solid ring, both in Monocyclica and Dicyclica. IBB may fuse with 

 the proximal columnal in Flexibilia, thus forming a pseudomonocyclic 

 type. BB may be overgrown by and incorporated with the ER, as 

 in JEugeniacrinus. 



The symmetry of the base is modified by the pi'esence of anals. 

 An anal resting on the basal circlet causes one of the BB to double 

 in width, so that the base becomes hexagonal instead of pentagonal. 

 Thus the quadripartite base comes to consist of a post, and ant. 

 large plate, and two small lateral plates (Fig. 4, v). These tend to 

 approximate in size. In Xenocrinus interbrachials as well as anals 

 come down between EK, so that the BB are nearly equal iu size, 



