VERTEBRAL COLUMN. 
4.35 
Vertebral Column. 
The upper part of the vertebral column of 
Pentacrinites is constructed on principles analo- 
gous to those already described in the upper part 
of the column of the Encrinite.* 
All the joints of the column, when seen trans- 
versely, present various modifications of penta- 
gonal star-like forms ; hence their name of As- 
teriae, or star-stones. 
These transverse surfaces are variously covei’ed 
'vith a succession of teeth, set at minute intervals 
from one another, and locking into the interstices 
between corresponding teeth on the surface of 
the next vertebrae, they are so disposed as to ad- 
wiit of flexure in all directions, without risk of 
dislocation, f 
As the base or root of Pentacrinites was usu- 
* The columnar joints of the Briarean Pentacrinite are dis- 
posed in pieces alternately thicker and thinner, with a third and 
®till thinner joint interposed between every one of them. PI. 53. 
^'gs. 8, and 8% a. b. c. The edges of this thinnest joint appear 
Externally only at the angles of the column ; internally they en- 
^E-rge themselves into a kind of intervertebral collar, c. c. c. 
A similar alternation in the joints of the Pentacrinite’s sub- 
^ogularis is represented in PI. .52. Figs. 4 and 5. 
t The ranges of tubercles upon the exterior surface of each 
joint in the fragments of columns, PI. 52. Figs. 7. 9. 11. mark 
origin and insertion of muscular fibres, by which the move- 
nient of every joint was regulated. At every articulation of the 
yertebrse, we see also the mode in which the crenated edges lock 
'nto one another, combining strength with flexibility. In PI. 52, 
