244 PALAEONTOLOGY 



this gives, with the canals, a pattern which is fairly 

 characteristic of the genus Cupressocrinus. During life 

 the slight amount of flexibility of the organic tissue of 

 each joint gave a suitable degree of flexibility to the 

 whole stem. 



The crown contained the essential vital parts of the 

 animal. It is divided into the theca or calyx, which 

 is the " body " of the animal and directly articulated to 

 the stem, and five arms which, when the animal was 

 feeding were spread out widely, but in the fossil are 

 usually found tightly closed up. The part of the calyx 

 which is visible when the arms are closed is called the 

 dorsal cup; it is the ventral surface which is hidden. 

 The dorsal cup consists of two circlets of five plates each 

 and a single pentagonal plate next to the stem. This 

 last is regarded (by analogy with other crinoids) as a 

 fused third circlet, the five plates of which if separate 

 would be called infrabasals ; the five plates in the circlet 

 next to it are called basals ; those in the upper circlet, 

 radials. The straight lines along which adjacent plates 

 meet are termed sutures. Radials and basals alternate in 

 position, and if the infrabasals were separate they would 

 alternate with the basals. Because of the presence of 

 (fused) infrabasals as well as basals, this crinoid is said 

 . to be dicyclic. In many crinoids the top columnal 

 directly joins the basals : they are monocyclic. 



The five radii which can be drawn from the centre of 

 the calyx through the middle of each radial plate are 

 called perradii, and organs such as the arms which lie 

 symmetrically upon them are said to be perradial in posi- 



