324 Mr. F. A. Bather on British Fossil Crinoids. 



logically by the right posterior radial and anal plate, the 

 other in the growing animal over the whole surface." Their 

 remarks of three years back on Hybocrinus are now modified ; 

 and they show that this genus possessed a true ventral sac, 

 though a very minute one. This organ is also present in 

 Ilybocystis. It is therefore probable that it was also present 

 in Hoplocrinus and Baerocrinus, though not preserved in the 

 few specimens known ; Wachsmuth and Springer do indeed 

 deny its presence, but, as they do not appear to have seen the 

 specimens, their mere negative statement is no more than an 

 opinion. It is unnecessary to give a further analysis of 

 Wachsmuth and Springer's views as they at present stand, 

 for they can be easily gathered from the diagrams. The 

 most debatable points only have been alluded to, and on 

 these we may summarize their position as follows : — 



(1) Azygos plate (Az) a primitive element of dorsal cup. 



(2) Anal ( x ) and right posterior radial derived from azygos 



plate. 



(3) Anal of Antedon not homologous with any plate of the 



Fistulata but an embryonic interradial. 



(4) Stages of evolution are : — 



(a) Baerocrinus, 4 Radials and I Azygos (fig. 4). 



(b) Hoplocrinus, large Azygos and small right posterior 



Radial (figs. 1 & 2). 



(c) Hybocrinus, large Azygos, right posterior Radial in- 



creasing, and x developed (fig. 3) . 



(d) Iocrinus, Radial growing larger at expense of Azygos, 



and here has absorbed x (fig. 5) . 



(e) Heterocrinus and Ectenocrinus, Azygos diminishing, x 



here removed from it (figs. 6 & 7). 



(f) Dendrocrinus, Radial still larger, Azygos smaller and 



touching x (fig. 15). 



(g) Homocrinus, Radial larger, Azygos smaller, x larger 



(fig. 16). 



Here is a dichotomy ; one branch continues : — 



(h) Botryocrinus, Radial nearly normal, Azygos small, x 



large (fig. 30). 

 (i) Gyathocrinus, Radial normal, Azygos entirely absorbed 



by Radial, x very large (fig. 20). 

 (j) Achradocrinus, the same, but x becoming absorbed 



(fig- 22). 

 (k) Codiacnnus, Radial normal, Azygos and x entirely 

 absorbed (fig. 23). 



