246 PALAEONTOLOGY 



In the case of Cupressocrinus the mouth is central and 

 appears as a large circular opening between five large 

 plates, interradial in position. Probably the actual 

 mouth was smaller, and other smaller plates surrounded 

 it. The five large plates are called orals or deltoids : one 

 of them differs from the rest in being deeply notched by 

 the anus. This is the only disturbance of radial 

 symmetry in the crown of Cupressocrinus, and it marks out 

 the odd radial as posterior in position ; the two adjacent 

 to it as postero-lateral and the other two as antero- 

 lateral. The margins of the orals that are in contact 

 with their neighbours bear semi-oval notches, which com- 

 bine into five pear-shaped perforations corresponding to 

 the middle line of each arm. Along these were continued 

 the five food-grooves that occupy the middle ventral line 

 of the arms. As the edges of the five orals meet around 

 the mouth the food-grooves must have finally entered the 

 mouth by tunnels, and the pear-shaped openings them- 

 selves may have been converted into tunnels by small 

 overlapping covering-plates (ambulacrals}. 



Each arm consists externally (dorsally) of a single row 

 of large brachial plates, tapering to the free end. In- 

 ternally (ventrally) they have a wide and deep groove, 

 with a row of pinnules (like miniature arms) along each 

 side, and a series of small plates (ambulacrals} arching 

 over it and converting it into a tunnel. This groove, in 

 life, was ciliated and constituted the food-groove or 

 ambulacrum, along which microscopic food was wafted 

 towards the mouth. The pinnules were similarly grooved 

 and ciliated, and served as tributaries. Thus the method 



