97 
At the Bass (PL. v1. 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-mnentioned muscular fibres, 
Avxintary Sipe Arms* (Pt. Actinocrinites levis fig. 28, to 39.) proceed 
at irregular distances occasionally from the column. They have their origin 
in a smal! 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 Auxiliary side arms of Actinocrinites levis are conformable to this species, hence I 
refer to that plate. 
oO 
