Figs. 1, 2.; Pl. 52. Fig. 3.; and Pl. 53.) will give a more accurate idea than can be conveyed by verbal descriptions.[1]
Vertebral Column.
The upper part of the vertebral column of Pentacrinites is constructed on principles analogous to those already described in the upper part of the column of the Encrinite.[2]
All the joints of the column, when seen transversely, present various modifications of pentagonal star-like forms; hence their name of Asteriæ, or star-stones.
These transverse surfaces are variously covered with a
- ↑ Pl. 51 represents s single specimen of Briarean Pentacrinite, which stands in high relief upon the surface of a slab of Lias, from Lyme Regis, almost entirely made up of a mass of other individuals of the same species. The arms and lingers are considerably expanded towards the position they would assume in searching for food. The side-arms remain attached to the upper portion only of the vertebral column.
At Pl. 53. Fig. 1 and 2 represent two other specimens of the same species, rising in beautiful relief from a slab, which is composed of a congeries of fragments of similar individuals. The columns of these specimens, Fig. 2, a, show the side-arms rising in their natural position from the grooves between the angular projections of the Pentagonal stem. At Pl. 52, Fig. F (a). F (b). are seen the costal plates surrounding the cavity of the body; at H, the Scapulæ, with the arms and lingers proceeding from them to the extremities of the tentacula.
At Pl. 53. Fig. 3. exhibits the side-arms rising from the lower part of a vertebral column, and entirely covering it. Fig. 4. is another column, on which, the side-arms being removed, we see the grooves wherein they articulated with the alternate vertebræ. Fig. 5. exhibits a portion of another column slightly contorted.
- ↑ The columnar joints of the Briarean Pentacrinite are disposed in pieces alternately thicker and thinner, with a third and still thinner joint interposed between every one of them. Pl. 53. Figs. 8, and 8a, a. b. c. The edges of this thinnest joint appear externally only at the angles of the column; internally they enlarge themselves into a kind of inter vertebral collar, c. c. c.
A similar alternation in joints of the Pentacrinites sub-angular is is represented in Pl. 52. Figs. 4 and 5.
