424 LILY ENCRINITE. 
varieties of form and contrivance which occur in 
the cokimn of a single species of Encrinite, may 
serve as an example of analogous arrangements 
in the columns of other species of the family of 
Crino'ideans, (see PL 47. Figs. 1, 2, 5, and PI. 49. 
Fig. 4 to Fig. 17).* 
The name of Entrochi, or wheel stones, has 
with much propriety been applied to these insu- 
lated vertebrae. The perforations in the centre 
of these joints aftbrding a facility for stringing 
them as beads, has caused them, in ancient 
times, to be used as rosaries. In the northern 
hollow cones, like the intervertebral cavities in the back of a fish, 
and to be, like them, subsidiary to the flexion of the column ; 
they probably also formed a reservoir for containing the nutritious 
fluids of the animals. 
The various kinds of Screw stone so frequent in the ciiert of 
Derbyshire, and generally in the Transition Limestone, are casts 
of the internal cavities of the columns of other species of Encri- 
niteS; in which the cones are usually more compressed than in 
the column of the E. moniliformis. 
* At PI. 49, Fig. 4 is a vertical section of Fig. 3, being a 
portion taken from near the summit of the column, where the 
greatest strength and flexure were required, and where also the 
risk of injury and dislocation was the greatest ; the arrangement 
of these vertebrse is therefore more complex than it is towards 
the base, and is disposed in the following manner (see Fig. 4). 
The vertebrae, a. b. c. are alternately wider and narrower ; the 
edges of the latter, c. are received into, and included within, the 
perpendicularly lengthened margin of the wider, a. b. ; the outer 
crenulated edge of the narrower included vertebrae, articulate 
with the inner crenulated edge of the wider vertebrae, which thus 
surround them with a collar, that admits of more oblique flexion 
