76 
joint to the inferior of that contiguous to it, and their firm articulation by means 
of the transverse ridge, the animal was capable of producing through its muscu- 
lar integument, a kind of twisting, vermicular spiral motion (Pr. rr. fig. 1. 18. 
1080.) shortening the column by com pressing the muscle between the two calcare- 
reous plates of each joint, and elevating it by untwisting and restoring it to its 
former elasticity. Itis evident that this motion would, in many ways, render the 
lateral motion of the superior part of the animal on its column 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 (Pr. ir. fig. 21. to 30.) 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. (Pr. r. 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 (Pr. rr. 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. (Pr. 11. fig. 52. to 56.) The columnar joints are sometimes 
rendered unshapely by exuded calcareous matter (Pr. 11. 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. 
(Pr. 11. 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. (Pu. rx. fig. 54.) 
The AuxiLrarY Sipe Arms. From the ends of the greater diameter of the 
elliptical columnar joints proceed the auxiliary side arms ) 11. fig. 32. and 
38.) formed of numerous round joints, perforated in the centre, and adhering: 
by surfaces striated in radii. 1 have specimens in my collection (Pr. 11. fig. 
39. to 44. fig. 47, to 51.) which from their analogy to some side arms still at- 
tached to the column, I consider as continnations of the same. They are re- 
‚warkably neat, round, and prove their former muscularity by the contraction 
