102 POTERIOCRINTD^ EXTRACRINUS. 



probable means. Thus by the mutual disposition and mutual accommodation of 

 uneven surfaces the column in its most flexible part was as well secured from injury 

 as if each of the joints had been dovetailed into those above and below it, while at the 

 same time its perfect flexure was provided for. 



The articulating facets of the joints undergo various modifications in accordance 

 with the change in the disposition of the ditierent parts of the column, but each 

 surface of articulation is furnished with five double series of crenated star-shaped radii, 

 which are more or less expanded according to the position they occupy in the column. 

 These beautiful crenated stellular markings were admirably adapted to impart both 

 strength and flexibility to the tall column, the office of which was to move laterally 

 in every direction to enable the animal to search for its food, by altering its position 

 as far as its attached state rendered it necessary or possible. The aperture through the 

 centre of all these ossicula formed a continuous canal from the base to the summit of 

 the column, and which Miller considered was for the passage of an alimentary canal. 

 Be this as it may, the perforation is extremely minute, and appears to be of equal size 

 throughout the column, which there is reason to suppose was several feet in length. 



The Auxiliary Side Arms, or Claspers. — These delicate lateral appendages to 

 the column, are so numerous that in a full grown specimen they amount to a thousand 

 or fifteen hundred. At and near the summit of the column every thick joint forms a 

 point of attachment for several of them, but they are not articulated with their longest 

 diameters coincident with the axis of the column, as represented by Miller. 'I'he 

 articulating bases are arranged alternately right and left at an angle of about 45° from 

 the true columnar axis, as represented in PI. 12, fig. i. By this contrivance they are 

 less crowded than if the articulations were perpendicular to the column. The joints 

 composing the lower part of the side arms are thin elliptical, or depressed, ovate bodies, 

 approaching to lozenge shape, and which are often carinated at each end of their longest 

 diameters. At a short distance from the point of attachment they gradually decrease 

 ill size and become more rotund in form until they terminate in an obtuse point. 



Lower down the column the side arms are less numerous than at the summit, so 

 that this crinoid was furnished with claspers in an inverse ratio to most other genera. 

 Tn thePoteriocrines and Actinocrines the side arms increase in number towards the base 

 of attachment, the upper portion of the column being destitute of them ; but in this 

 species of Extracrinus the exact reverse of this is observed, the lower part of the 

 column being most free from them, while towards the summit they are so closely set 

 as to completely conceal all but the prominent angles of the pentagonal column, or 

 even these portions are not always clearly perceptible when the side arms are closely 

 folded in a compact fasciculus around the column. They always emanate from the 



