THE ECHINODERMATA 255 



is five-sided, and the columnals show a very striking 

 pattern. Apiocrinm (Fig. 71, h, Jur.) has a cylindrical 

 stem, which expands at its upper end, so that its outline 

 passes imperceptibly into that of the crown ; the infra- 

 basals are not recognizable (pseudo-monocyclic or crypto- 

 dicyclic), the arms branch into two very quickly, and 

 their lower part is incorporated in the cup. Beautiful 

 specimens have been found, and fragments are common, 

 in the Bradford Clay of Bradford-on-Avon. In Bour- 

 gtiettcrinus (Fig. 70, g, Cret.) most of the columnals have 

 the same peculiarity as in Platycrinus (ante, p. 251), but are 

 longer and more barrel-shaped ; towards the crown they 

 are round and the proximal one is as wide as the cup, 

 which shows five basals and five radials : the arms are 

 rarely preserved. Antedon or Comatula (Jur.-Rec.) is 

 fixed by a stem in its earliest youth, but afterwards 

 becomes free-swimming. Another free-swimming form is 

 Saccocoma, in which the radials form nearly the whole 

 cup ; the arms are bifurcated and the ten branches 

 often spirally coiled, the brachials bearing wing-like 

 expansions. This is abundant in the Upper Jurassic 

 lithographic stone of Solnhofen, Bavaria, and pyritized 

 plates are found at the same horizon in the English 

 Kimmeridge Clay, recognizable by the coarse meshwork 

 of stereom. 



The Cystidea are a somewhat heterogeneous group, 

 consisting of the most primitive forms, in which gradual 

 development of five-rayed symmetry can to some extent 

 be traced. 



Echinosphaera aurantium (Fig. 72) is very abundant 

 in the Cystid Limestone of the Ordovician of Sweden, 

 erratics of which, carried by ice, are common on the 

 plains of North Germany. It is an almost spherical 



