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 admit of flexure in all directions, without risk of dislocation.[1]
As the base or root of Pentacrinites was usually fixed to the bottom of the sea, or to some extraneous floating body, the flexibility of the jointed column, which forms the stem, was subservient to the double office, first, of varying, in every direction, the position of the body and arms in search of food, and secondly, of yielding, with facility, to the course of the current, or fury of the storm, swinging, like a vessel held by her cable, with equal ease in all directions around her moorings.
The Root of the Briarean Pentacrinite was probably slight, and capable of being withdrawn from its attachment.[2] The absence of any large solid Secretious, like
- ↑ The ranges of tubercles upon the exterior surface of each joint in the fragments of columns, Pl. 52. Figs. 7. 9. 11. mark the origin and insertion of muscular fibres, by which the.movement of every joint was regulated. At every articulation of the vertebrae, we see also the mode in which the crenated edges lock into one another, combining strength with flexibility. In Pl. 52, Figs. ll. and 13, the Vertebræ (d.) present five lateral surfaces of articulation, whereby the side-arms were attached to the vertebral column at distant intervals, as in the Pentacrinus Caput Medusæ, Pl. 52. Fig, 1.
The double series of crenated surfaces, which pass from the centre to the points of each of the five radii of these star-shaped vertebra, Pl. 52. Fig. 6. to 17.; and Pl. 53. Figs. 9. to 13, present a beautiful variety of arrangements, not only in each species, but in different parts of the column of the same species, according to the degree of flexion which each individual part required.
- ↑ Mr. Miller describes a recent specimen of Pentacrinus Caput Medusæ, as having the joints next to the base partially consolidated, and admitting but little motion, where little is required; but higher up, the joints become thinner, and are disposed alternately, a smaller and thinner joint succeeding a larger and thicker, to allow a greater freedom of motion, till near the apex this change is so conspicuous, that the small ones resemble thin leather-like inter positions. He also observed traces of
