76 



joint to the inferior of that contiguous to it, ami their firm articulation by means 

 of the transverse ridge, the animal was capahle of producing through its muscu- 

 lar integument, a kind of twisting, vermicular spiral motion (PL. n. fig. 1. 18. 

 to 20.) shortening the column by compressing the muscle between the twoealcare- 

 reous plates of each joint, and elevating it by untwisting and restoring it to its 

 former elasticity. It is evident that this motion would, in many ways, render the 

 lateral motion of the superior part of the animal on its coluom less necessary; 

 and since the attachment of the pelvis to the column appears to have been 

 slight, and might have been endangered by such a motion, this may have been 

 the object of the contrivance. When the two hard elliptical surfaces, with their 

 elevated ridges are placed obliquely to each other, the joint evidently ap- 

 pears to be considerably compressed, and has a slight four-cornered appearance, 

 having the angles rounded ; while along the middle of the exterior circumference 

 the intermediate muscle is pressed out (PL. n. fig. 2 1. to ,10.) it forms a smooth 

 rim, which is sometimes studded at intervals with tubercles occasionally 

 elongated into a kind of pointed feelers, giving the joint some resemblance to 

 the rowel of a spur. (PL. n. fig. 25. to 26.) In general the protruding feelers 

 are longer near the ends of the greater diameter of the elliptical joint, sometimes 

 assuming an inclining direction (PL. n. fig. 28. and 30.) and having an indented 

 summit. The alimentary canal of the column is small, probably because the 

 muscular integument was more vascular. 



The BASE. (PL. n. fig. 52. to 56.) The columnar joints are sometimes 

 rendered unshapely by exuded calcareous matter (PL. n. fig. 52. and 

 53) which, near the base, appears to have partially surrounded those 

 numerous muscular fibres which served the animal as organs of attachment. 

 (PL. ii. fig. 55. and 56.) In specimens where these fibres are decayed and lost, 

 many cavities are left behind in the indurated matter, demonstrating their 

 points of insertion. (PL. n. fig. 54.) 



The AUXILIARY SIDE ARMS. From the ends of the greater diameter of the 

 elliptical columnar joints proceed the auxiliary side arms (PL. n. fig. 32. and 

 33.) formed of numerous round joints, perforated in the centre, and adheriiv 

 by surfaces striated in radii. I have specimens in my collection (PL. n. tig. 

 39. to 44. fig. 47. to 51.) which from their analogy to some side arms still at- 

 tached to the column, I consider as continuations of the same. They ai;e re- 

 markably neat, round, and prove their former muscularity by the contraction 



