97 



At the BASE (PL. vi. fig. 23. to 27.) of the column, the joints articulate simi- 

 larly, but are irregularly waved; sometimes only extending for half the diame- 

 ter of the column, then bevelling and insinuating under those near them. Their 

 external circumference is here marked by small perforations, from whence 

 muscular fibres appear to have protruded. A calcareous secretion has some- 

 times exuded on this part, surrounded the column, and concealed its articu- 

 lation, forming an inorganic investing mass, as appears on examining transverse 

 fractures. This exudation sometimes swells out into large knobs, which are 

 also perforated for the insertion of the before-mentioned muscular fibres. 



AUXILIARY SIDE ARMS* (PL. Actinocrinites laevis fig. 28. to 39.) proceed 

 at irregular distances occasionally from the column. They have their origin 

 in a small tubercle (fig. 28.) placed at the articulation of two joints, containing 

 a vessel branching off from the alimentary canal ; as this tubercle increases and 

 extends, it forms joints in its progress articulating to each other by striated 

 surfaces. The joints near the column produce in it a round concave impres- 

 sion, and as the side arms increase in size, this impression expands until it 

 spreads over five or even more columnar joints (fig. 31. to 33.) In the progress 

 of its growth a very remarkable circumstance takes place; the nourishing vessel 

 of the side arm, which appears in the early stage of growth to communicate, as 

 has been said, with the alimentary canal by passing along the interval between 

 two columnar joints (fig. 28. 29. to 36.) may now be distinctly traced (in some 

 well preserved specimens), passing through the middle of the substance of a 

 single columnar joint. (Fig. 31 . and 39.) If this observation can be relied on 

 (which the minuteness of younger specimens may render doubtful) it distinctly 

 proves that a new columnar joint (that namely through whose substance the 

 channel of the vessel now passes), has been gradually formed and interposed 

 during the growth of the animal. 



The mechanism of the joints of the side arms, where these insert into the 

 column, is well worthy of notice, particularly in old specimens. In the earlier 

 stage of their formation, the side arms being very short, and having then little 

 weight, a less firm mode of adhesion to the column than becomes requisite at 

 a subsequent period, being then sufficient, we do not find more than one joint 



* The An: iliary side arms of Actiuocrinites laevis are conformable to this species, hence I 

 refer to that plate. 



