438 MECHANISM OF SIDE ARMS. 
Side Arms, 
The Side Arms become gradually smaller to- 
wards the upper extremity of the column. In 
the P. Briareus (PI. 52, Fig. 3. and PI. 53, Fig. 
1. and 3.) these amount to nearly a thousand in 
number.^ The numerous side arms of the Bria- 
rean Pentacrinite, when expanded, would act as 
auxiliary nets to retain the prey of the animal, 
and also serve as hold-fasts to assist it in ad- 
hering to the bottom, or to extraneous bodies. 
In agitated water they would close and fold 
themselves along the column, in a position which 
would expose the least possible surface to the 
groups, (like modern barnacles), to the masses of floating wood, 
which, together with them, were suddenly buried in the mud, 
whose accumulation gave origin to the marl, wherein this curious 
compound stratum of animal and vegetable remains is imbedded. 
Fragments of petrified wood occur also in the Lias, having large 
groups of Mytili, in the position that is usually assumed by recent 
mytili, attached to floating wood. 
* If we suppose the lower portion of the specimen, PL 53, Fig. 
2. a. to be united to the upper portion of the fractured stem, Fig. 
3, we shall form a correct idea of the manner in which the column 
of this animal was surrounded with its thousand side arms, each 
having from fifty to a hundred joints, PL 53, Fig. 14. The 
number of joints in the side arms gradually diminishes towards 
the top of the vertebral column ; but as one of the lowest and 
largest (PI. 53, Fig. 14.) contains more than a hundred, we shall 
be much below the reality in reckoning fifty as their average 
number. 
Each of these joints articulates with the adjacent joint, by 
processes resembling a mortice and tennon ; and the form both of 
