H2 THE CRINOIDEA 



the change from Dicyclic to Monocyclic may have taken place, 

 not by compression and atrophy, but by torsion and fusion. Many 

 monocyclic genera of Ordovician and Silurian age have some 

 radials (usually r. post., r. ant., and 1. ant. RR) transversely bisected ; 

 the upper part is called "super-radial" (Rs) ; the lower part, 

 " inferradial " (Ri), (see Fig. LVIIL). Now, in some dicyclic 

 genera (e.g. Ottawacrinus, Fig. XCVL), perhaps in consequence 

 of the introduction of fresh plates in the anal interradius, the 

 radials are shifted to right and left so as to lie almost vertically 

 above the basals. The suggestion then is that the inferradials and 

 basals of Monocyclica represent basals and infrabasals respectively 

 of Dicyclica. If then the Rs and Ri fuse, a truly monocyclic type 

 is produced with one circlet of BB and one of RR. One obvious 

 objection to this theory is the presence in many Dicyclica of a 

 plate (the radianal, RA), which is now generally regarded as a 

 slightly modified inferradial (r. post. Rz), (Fig. XXVI.). Other 

 objections to this and similar views, based by P. H. Carpenter 

 (1878) on alleged homologies with the apical system of Echinoidea, 

 have, it is true, been somewhat discredited by modern embryological 

 and palaeontological research. Nevertheless, for the present the 

 gulf between Monocyclica vera and Dicyclica is unbridged, and 

 must be recognised in classification. 



The dorsal cup of a simple crinoid consists of two or three 

 circlets, but there is often a tendency for the proximal brachials to 

 be so joined to the radials and to one another as to form part of 

 the cup (Fig. XIX.). There is, however, a supposed 

 \\Br morphological distinction between these "fixed 

 brachials " (Br) and the radials : the latter are de- 

 veloped in the Antedon larva as expanded sieve- 

 like films ; but all brachials begin as " imperfect 

 rings, which soon become filled up with lengthening 

 fasciculated tissue" (P. H. Carpenter, 1884; see Fig. 

 XIX.). In a form where many brachials enter the 

 dorsal cup, it is convenient to have a common term 

 FIG. xix. f r the primitive elements of the cup (IBB, BB, 

 R, radial, crib- RR) : some call them the "apical system," postu- 

 riform stereom ; lating a homology with the plates so called in 



IBr, primibrach, 8 /, . \ . , 



fasciculate ste- Echinoidea ; the old term patina is shorter and safer. 

 doT^/Ma, x*75! To understand the extension of the cup beyond 

 by as w i a ca tlie P atina > & is necessary first to study the Arms 

 penter.) O r Bracliia (Fig. XX.). These are rarely single. 



The first step in advance is a bifurcation, con- 

 stantly repeated in a regular manner (regular dichotomy or 

 isotomy). Modifications of this occur through the suppression 

 of a bifurcation at definite points (irregular dichotomy or hetero- 

 tomy). In each half of the arm, the first branch on the right, 



