CHAP. ix.J THE DEEP -SEA FAUNA. 441) 



from the symmetry and compactness of the Apio- 

 crinid;e of the Jurassic period. 



The anchylosed ring of first radials is succeeded by 

 a tier of free second radials, which are united by a 

 straight syzygial suture to the next series the radial 

 axillaries. The surface of the funnel-shaped dilatation 

 of the stem, headed by the ring of first radials. is 

 smooth and uniform, and the second radials and 

 radial axilla ries present a smooth, regularly-arched 

 outer surface. The radial axillaries differ from the 

 corresponding joints in most other known crinoids 

 in contracting slightly above, presenting only one 

 articulating facet, and giving origin to a single arm. 

 The arms, which in the larger specimens are from 10 

 to 12 mm. in length, consist of a series of from about 

 twenty-eight to thirty-four joints, uniformly trans 

 versely arched externally, and deeply grooved within 

 to receive the soft parts. Each alternate joint bears 

 a pinnule, the pinnules alternating on either side of 

 the axis of the arm, and the joint which does not 

 bear a pinnule is united to the pinnule-bearing joint 

 above it by a syzygy : thus joints with muscular 

 connections and syzygies alternate throughout the 

 whole length of the arm. 



The pinnules, twelve to fourteen in number, con 

 sist of a uniform series of minute joints, united by 

 muscular connections. The grooves of the arms and 

 of the pinnules are bordered by a double series of 

 delicate round fenestrated calcareous plates, which, 

 when the animal is contracted and at rest, form a 

 closely imbricated covering to the nerve, and the 

 radial vessel with its delicate crecal tentacle3. The 

 mouth is placed in the centre of the disk, and radial 



G G 



