LILY ENCRINITE. 319 



ate variations both in the external and internal form and 

 dimensions of each part.* The varieties of form and con- 

 trivance which occur in the column of a single species of 

 Encrinite, may serve as an example of analogous arrange- 

 ments in the columns of other species of the family of Cri- 

 noideans, (see PI. 47. Figs. 1,2, 5, and PL 49. Fig. 4 to 

 Fig. 17.t) 



• The body (Pi. 49, Fig. 1) is supported by a long vertebral column at- 

 tached to the ground by an enlargement of its base (PI. 49, Fig. 2.) It is 

 composed of many cylindrical thick joints, articulating firmly with each other, 

 and having a central aperture, like the spinal canal in the vertebra of a quad- 

 ruped, through which a small alimentary cavity descends from the stomach 

 to the base of the column, PI. 49, Fig. 4, 6, 8, 10. The form of the column 

 nearest the base is the strongest possible, viz. cylindrical. This column is 

 inten-upted, at intervals, which become more frequent as it advances up- 

 wards, by joints of wider diameter and of a globular depressed form (Pi, 49, 

 Fig. 1, and Figs. 3, 4, a, a, a, a.) Near the summit of the column, (Pi. 49, 

 Figs. 3, 4,) a series of tliin joints, c, c, c, is jiluced next above and below 

 each largest joint, and between these two thin joints, there is introduced a 

 tliird series, b, b, b, of an intermediate size. The use of these variations 

 in the size of tiie interpolated joints was to give increased flexibility to that 

 ])art of the column, which being nearest to its summit required the greatest 

 power of flexion. 



At Plate 49, Figs. 6, 8, 10, are vertical sections of the columnar joints 

 5, 7, 9, taken near the base; and sliow the internal cavity of the column, to 

 be arranged in a series of double hollow cones, like the intervertebral cavi- 

 ties 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 nutri. 

 tious fluids of tiie animals. 



The various kinds of Screw stone so frequent in the chert of Derbyshire, 

 and generally in the Transition Limestone, are casts of the internal cavities 

 of the columns of other species of Encrinites, in which the cones are usually 

 more compressed than in the column of the E. moniliformis. 



t 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 and injury and dislocation was the 

 greatest; the arrangement of these vertebra 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 length- 



