35 



nerally coincide at the opposite extremities of the same joint, the longitudinal 

 axis of the one being placed obliquely when compared with that of the other, 

 as if the joint had been twisted round in opposite directions at its two extremi- 

 ties. (This is represented in fig. 18. by dotted lines.) I have never found a 

 single instance in which this obliquity was not observable, although there are 

 reasons for believing that when the animal was fully extended, the direction of 

 the opposite surfaces must have become coincident, as in the PLATYCRINITES. 

 This obliquity of the two elliptical surfaces, appears to have resulted from a 

 vermicular contraction extended from one joint to the other, through the 

 muscle surrounding the alimentary canal, which being inserted in the trans- 

 verse grooves, and acting on them like a screw driver, twisted them partially 

 round ; hence, from the compression thus produced, we see the circumference 

 of the joints sometimes barrelshaped (16.) or more or less round towards the 

 centre, while near the articulating surface it remains elliptical. The barrel- 

 shaped joints are thicker in proportion, and appear to have lost their soft carti- 

 laginous texture, having probably belonged to full grown animals ; this, indeed, 

 seems to be confirmed by some of the specimens exhibiting a formation in con- 

 centric rings, like that observable in Apiocrinites rotundus, and an exterior par- 

 tial separation. (16.) The alimentary canal is generally small, but appears to 

 have enlarged considerably in some parts of the column (22.) and in this case 

 elongated the joints containing it, and at the same time strengthened their adhe- 

 sion with the neighbouring joints by secreting and accumulating calcareous mat- 

 ter around it, (20. to 22.) In some specimens (20. to 22.) of this description, the 

 line of junction between two joints, instead of being as usual straight, be- 

 comes inflected and waved, apparently in consequence of having been acted 

 upon by a muscular contraction, and in one instance, the exuded calcareous 

 matter has completely obliterated the external mark of articulation between 

 two joints (19.) which have anchylosed therefore to each other. LATERAL 

 AUXILIARY ARMS (13.) appear to have proceeded occasionally from the column 

 at irregular intervals, the joints composing them are round, and probably 

 articulated by radiating surfaces, as is the case in PLATYCRINITES. 



The ENLARGED COLUMNAR JOINTS at the summit of the column, I appre- 

 hend to have articulated similarly to those of ordinary size, and in a section (5) 

 in my possession I have traced the alimentary canal passing through their 

 centre, enlarging in the second joint. 



. 



In the disposition and general figure of the joints (1. to 3.) forming the upper 



